See mum after swim gets certificate (7)
I believe the answer is:
diploma
'certificate' is the definition.
(academic certificate)
'see mum after swim' is the wordplay.
'see' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
'mum' becomes 'ma' (ma can mean mother).
'after' means one lot of letters go next to another.
'swim' becomes 'dip' (dip can mean a brief swim).
'lo'+'ma'='loma'
'loma' put after 'dip' is 'DIPLOMA'.
'gets' acts as a link.
(Other definitions for diploma that I've seen before include "Document conferring a qualification" , "Document conferring honour" , "documentation" , "Exam certificate" , "Paper showing examination success" .)