Puzzle firm close to London put together (7)
I believe the answer is:
confuse
'puzzle' is the definition.
(synonyms)
'firm close to london put together' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'close to' indicates one should take the final letters (the 'close' to the word).
'put together' becomes 'fuse' (fuse can mean to join).
The last letter of 'london' is 'n'.
'co'+'n'+'fuse'='CONFUSE'
(Other definitions for confuse that I've seen before include "Disconcert, befuddle" , "Puzzle" , "Perplex" , "Muddle, mix up" , "Bewilder" .)