Skin disease: woman’s source of annoyance, mostly (6)
I believe the answer is:
herpes
'skin disease' is the definition.
(I've seen this before)
'woman's source of annoyance mostly' is the wordplay.
'woman' becomes 'her' (her refers to a woman).
'source of annoyance' becomes 'pest' (I've seen this in another clue).
'mostly' means to remove the last letter (most but not all of the word).
'pest' with its final letter taken away is 'pes'.
'her'+'pes'='HERPES'
(Other definitions for herpes that I've seen before include "15 across (anag)" , "Viral disease of the skin" , "Sphere of virus disease" , "Virus disease causing blisters on skin" , "Skin disease - shingles, for instance" .)