Be of assistance when a small number suffer (5)
I believe the answer is:
'be of assistance' is the definition.
(I've seen this in another clue)
'a small number suffer' is the wordplay.
I cannot quite see how this works, but
'a' is present in the answer.
'number' could be 'v' (V is an example) and 'v' is present in the answer.
'suffer' could be 'ail' (ailing is a kind of suffering) and 'ail' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'when' acts as a link.
Can you help me to learn more?
(Other definitions for avail that I've seen before include "Be of help or advantage" , "Use to one's advantage" , "Medal (informal)" , "Be of value to" , "Make use, take advantage" .)