Member of force with unknown ape (4)
I believe the answer is:
copy
'ape' is the definition.
(synonyms)
'member of force with unknown' is the wordplay.
'member of force' becomes 'cop' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' means one lot of letters go next to another.
'unknown' becomes 'y' (algebra).
'cop'+'y'='COPY'
Can you help me to learn more?
(Other definitions for copy that I've seen before include "Borrow" , "Imitation or matter for printing" , "Imitation; matter to be printed" , "Text for an article" , "Material for publication" .)