Keep track of nickel deposit in engine (7)
I believe the answer is:
monitor
'keep track of' is the definition.
(I've seen this in another clue)
'nickel deposit in engine' is the wordplay.
'nickel' becomes 'Ni' (Ni is the chemical symbol for nickel).
'deposit in' means one lot of letters goes inside another.
'engine' becomes 'motor' (I've seen this before).
'ni' going into 'motor' is 'MONITOR'.
(Other definitions for monitor that I've seen before include "what a computer user may require" , "giant lizard" , "School pupil assigned responsibilities" , "in-class help" , "Lizard; display screen" .)