Make a strenuous effort (4,1,3)
I believe the answer is:
bust a gut
'make a strenuous effort' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bust a gut that I've seen before include "Try really hard (slang)" , "Make a strenuous effort" , "Really work extra hard" , "Make a huge effort (slang)" , "Give it everything" .)