A child, oddly sharp (4)
I believe the answer is:
acid
'sharp' is the definition.
(as in sharp-tasting)
'a child oddly' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
The alternate letters of 'child' are 'cid'.
'a'+'cid'='ACID'
(Other definitions for acid that I've seen before include "Sour-tasting" , "Bitter substance" , "Tart - keen - sort of house party" , "Hallucinogenic drug" , "Corrosive fluid" .)