What retainers do: go back to be accepted by family (4,2)
I believe the answer is:
keep in
'what retainers do' is the definition.
The definition suggests a singular noun but the answer is not.
'go back to be accepted by family' is the wordplay.
'go' becomes 'pee' (both can mean to urinate).
'back' says the letters should be written in reverse.
'to be accepted by' indicates putting letters inside (inserted letters are accepted).
'family' becomes 'kin' (synonyms).
'pee' written backwards gives 'eep'.
'eep' going inside 'kin' is 'KEEP IN'.
Can you help me to learn more?
(Other definitions for keep in that I've seen before include "Bottle up - shut up" , "Contain" , "punish a pupil" , "Restrain" , "Conceal" .)
