Support needed to hold C sharp (4)
I believe the answer is:
acid
'sharp' is the definition.
(I've seen this before)
'support needed to hold c' is the wordplay.
'support' becomes 'aid' ('aid' can be a synonym of 'support').
'needed to hold' indicates putting letters inside (to hold can mean to receive or contain).
'aid' placed around 'c' is 'ACID'.
(Other definitions for acid that I've seen before include "Vinegary" , "Sour tasting" , "Bitter - drug" , "Drug LSD (colloq.)" , "Polluted precipitation" .)