Seize an opportunity (4,3)
I believe the answer is:
make hay
'seize an opportunity' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for make hay that I've seen before include "work in a meadow" , "Have great fun" , "Do it while you can, when the sun shines" , "Seize the opportunity" , "produce fodder" .)