Engineer's staff get on (6)
I believe the answer is:
'engineer's' is the definition.
'staff get on' is the wordplay.
'staff' becomes 'man' (manning is a kind of staffing**).
'get on' becomes 'age' ('age' can be a synonym of 'get on').
(Other definitions for manage that I've seen before include "get on" , "Handle, cope with" , "Handle, oversee" , "Control, direct, run" , "Handle, run" .)