Decide to turn off before pit (9)
I believe the answer is:
determine
'decide' is the definition.
(I know that decide can be written as determine)
'turn off before pit' is the wordplay.
'turn off' becomes 'deter' (to deter someone is to turn them off something).
'before' means one lot of letters go next to another.
'pit' becomes 'mine' (mining pit).
'deter'+'mine'='DETERMINE'
'to' acts as a link.
(Other definitions for determine that I've seen before include "resolve" , "fix" , "Firmly decide" , "Decide - to settle" , "Dictate" .)