Become expert in microwave device, having invested time (6)
I believe the answer is:
master
'become expert in' is the definition.
(I've seen this in another clue)
'microwave device having invested time' is the wordplay.
'microwave device' becomes 'maser' (I've seen this in another clue).
'having invested' indicates putting letters inside.
'time' becomes 't' (abbreviation).
'maser' enclosing 't' is 'MASTER'.
(Other definitions for master that I've seen before include "Gain control of (something)" , "One has consummate skill" , "Maestro; teacher" , "Subdue, win over" , "An old one is a great European painter" .)