Fix on and turn off underwater explosive (9)
I believe the answer is:
determine
This could be a double definition.
'fix on' is the first definition.
(thesaurus)
'turn off underwater explosive' is the second definition.
'determine' can be an answer for 'turn' (thesaurus). I'm not certain of the 'off underwater explosive' bit.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for determine that I've seen before include "Firmly decide" , "Ascertain by research or calculation" , "Come to a decision" , "Decide - to settle" , "Dictate" .)