MP and Sue frolic in the foam (5)
I believe the answer is:
spume
'the foam' is the definition.
(spume is a kind of foam)
'mp and sue frolic' is the wordplay.
'and' means one lot of letters go next to another.
'frolic' indicates an anagram.
'mp' put after 'sue' is 'suemp'.
'suemp' with letters rearranged gives 'SPUME'.
'in' acts as a link.
(Other definitions for spume that I've seen before include "Foam on waves" , "Froth on waves" , "Sea foam, froth" , "Surf" .)