Engineering lead and diver unite for good (9)
I believe the answer is:
eternally
'for good' is the definition.
(I've seen this before)
'engineering lead and diver unite' is the wordplay.
'lead' says to take the initial letters.
'and' is a charade indicator (letters next to each other).
'diver' becomes 'tern' (type of diving bird).
'unite' becomes 'ally' (I've seen this in another clue).
The initial letter of 'engineering' is 'e'.
'e'+'tern'+'ally'='ETERNALLY'
(Other definitions for eternally that I've seen before include "for good" , "Nearly let out forever" , "all the time" , "constantly" , "Always and forever" .)