Let slip of a girl eat both crackers (4,3,4)
I believe the answer is:
miss the boat
'let slip' is the definition.
I know nothing about this answer so I can't tell whether this works.
'a girl eat both crackers' is the wordplay.
'a girl' becomes 'miss' (term of address for an unmarried woman).
'crackers' is an anagram indicator (informal term for mad).
'eatboth' with letters rearranged gives 'theboat'.
'miss'+'theboat'='MISS THE BOAT'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for miss the boat that I've seen before include "Blow opportunity" , "Lose opportunity" , "Don't take a chance" , "as tardy tars may do" , "Fail through lateness" .)