Where girl may go, boy almost stops (6)
I believe the answer is:
ladies
'where girl may go' is the definition.
(ladies toilets)
'boy almost stops' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'almost' means to remove the first letter.
'stops' becomes 'dies' (as in a machine stopping or dying).
'dies' with its initial letter removed is 'ies'.
'lad'+'ies'='LADIES'
(Other definitions for ladies that I've seen before include "9th" , "Women of rank in the British peerage" , "Ideals for genteel women" , "Wives of knights" , "Women, of refinement perhaps" .)