Engineers try hard to gain control (8)
I believe the answer is:
restrain
'control' is the definition.
(restraining is a kind of controlling)
'engineers try hard' is the wordplay.
'engineers' becomes 'RE' (abbreviation for Royal Engineers).
'try hard' becomes 'strain' (I've seen this in another clue).
're'+'strain'='RESTRAIN'
'to gain' acts as a link.
(Other definitions for restrain that I've seen before include "Keep in check" , "bottle up" , "Control - check" , "Tie up - arrest" , "Keep under control" .)