Sharp, cold and thin (5)
I believe the answer is:
clean
'sharp' is the definition.
(synonyms)
'cold and thin' is the wordplay.
'cold' becomes 'C' (eg on taps).
'and' is a charade indicator (letters next to each other).
'thin' becomes 'lean' (synonyms).
'c'+'lean'='CLEAN'
(Other definitions for clean that I've seen before include "Not soiled, free from dirt" , "Squeakily hygienic?" , "Laundered" , "Honest" , "far from blue" .)