Bitter substance girl put in can (6)
I believe the answer is:
tannin
'bitter substance' is the definition.
(I've seen this before)
'girl put in can' is the wordplay.
'girl' becomes 'ann'.
'put in' is an insertion indicator.
'can' becomes 'tin' ('tin' can be a synonym of 'can').
'ann' going within 'tin' is 'TANNIN'.
(Other definitions for tannin that I've seen before include "Substance used in treating leather" , "Compound from bark or tea, used for dyeing" , "It occurs in tea and grapes" , "Compound used in dying also in tea" , "Plant substance used in, eg, leather-making, medicine and red wine" .)