Gestured ‘I’m in the sea!’ (5)
I believe the answer is:
mimed
'gestured' is the definition.
Although both the answer and definition are past participle verbs, I can't see how they can define each other.
'i'm in the sea' is the wordplay.
'in' indicates putting letters inside.
'the sea' becomes 'med' (short for Mediterranean).
'im' inserted inside 'med' is 'MIMED'.
Can you help me to learn more?
(Other definitions for mimed that I've seen before include "made a charade" , "Took off <" .)