Pull in specialised troops north of zone (7)
I believe the answer is:
retract
'pull in' is the definition.
(retracting is a kind of pulling in)
'specialised troops north of zone' is the wordplay.
'specialised troops' becomes 'RE' (Royal Engineers in the British Army).
'north of zone' becomes 'tract' (I can't explain this - if you can you should give a lot more credence to this answer).
're'+'tract'='RETRACT'
(Other definitions for retract that I've seen before include "Withdraw as unjustified" , "Cancel" , "Withdraw one's statement" , "Take back (statement)" , "Recant" .)