Make the best use of (8)
(Other definitions for optimise that I've seen before include "Tune up (S, not Z)" , "Make as effective or favourable as possible" , "Render best" , "Make the best of (something)" , "best possible result" .)
'make the best use of' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for maximise that I've seen before include "Make the best use of" , "Make as big as possible" , "Make as large as possible" .)