Small undersized person’s a pain on the run (6)
I believe the answer is:
stitch
'run' is the definition.
The answer and definition can be both to do with contact as well as being verbs in their base form.
Perhaps you can see an association between them that I can't see?
'small undersized person's a pain on the' is the wordplay.
I cannot really understand how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is found in the answer.
'pain' could be 'itch' (I have seen 'Irritating pain ' mean 'itch' so perhaps 'pain' could also mean 'itch') and 'itch' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stitch that I've seen before include "''A . . . . . . in time saves nine'' (6)" , "Having very little to wear" , "Perform final act in theatre" , "One in time is said to save nine" , "Interlinking loop of yarn" .)