Potential pain in side of mouth or nose (5)
I believe the answer is:
thorn
'potential pain in side' is the definition.
(as in being a thorn in someone's side)
'of mouth or nose' is the wordplay.
'of' says the answer is hidden in the clue.
'THORN' can be found hidden inside 'mouth or nose'.
(Other definitions for thorn that I've seen before include "Sharp point on a shrub" , "Sharp woody projection" , "Pointed extension of leaf" , "Sharp point on plant" , "Spine on some plants" .)