At least one drop should help this fashion lacking length (4)
I believe the answer is:
stye
'at least one drop should help this' is the definition.
(a stye might be treated with eye drops)
'fashion lacking length' is the wordplay.
'fashion' becomes 'style' (synonyms).
'lacking' indicates named letters should be taken away.
'length' becomes 'l'.
'style' with 'l' removed is 'STYE'.
(Other definitions for stye that I've seen before include "Eye blemish" , "makes viewer sore" , "dump" , "Observer's problem" , "producing eyesore" .)