Join one end, firstly (5)
I believe the answer is:
unite
'join' is the definition.
(synonyms)
'one end firstly' is the wordplay.
'one' becomes 'unit' (synonyms).
'firstly' says to take the initial letters.
The first letter of 'end' is 'e'.
'unit'+'e'='UNITE'
(Other definitions for unite that I've seen before include "Group of workers" , "pool resources" , "Connect" , "Gather together as one" , "Make one" .)