Marine shook a leg to get in the picture (7)
I believe the answer is:
pelagic
'marine' is the definition.
(I've seen this before)
'shook a leg to get in the picture' is the wordplay.
'shook' indicates an anagram.
'to get in' is an insertion indicator.
'the picture' becomes 'pic' (short for picture).
'a'+'leg'='aleg'
'aleg' with letters rearranged gives 'elag'.
'elag' going within 'pic' is 'PELAGIC'.
(Other definitions for pelagic that I've seen before include "of the ocean" , "Inhabiting the deep" , "Sea" , "Like an albatross" , "Marine" .)