Bring into operation deal, perhaps accepting unknown backing (5)
I believe the answer is:
exert
'bring into operation' is the definition.
(I've seen this before)
'deal perhaps accepting unknown backing' is the wordplay.
'deal perhaps' becomes 'tree' (I can't justify this - if you can you should believe this answer much more).
'accepting' means one lot of letters goes inside another (inserted letters are accepted).
'unknown' becomes 'x' (algebra).
'backing' shows that the letters should be reversed in order.
'tree' enclosing 'x' is 'trexe'.
'trexe' written backwards gives 'EXERT'.
Can you help me to learn more?
(Other definitions for exert that I've seen before include "Apply (pressure)" , "Bring into use, say strength" , "Bring (force) to bear" , "Act upon" , "Exercise, wield" .)