Lark about, romp (6)
I believe the answer is:
cavort
I believe this is a double definition.
'lark about' is the first definition.
(synonyms)
'romp' is the second definition.
(I know that romp can be written as cavort)
(Other definitions for cavort that I've seen before include "Play boisterously" , "be frolicsome" , "Frisk and frolic" , "'Lark about, romp (6)'" , "Frolic, gambol" .)