Joining the firm at this point, form a union (6)
I believe the answer is:
cohere
'form a union' is the definition.
The definition and answer are not the same part of speech.
'joining the firm at this point' is the wordplay.
'joining' says to put letters next to each other.
'the firm' becomes 'co' (a firm is a company).
'at this point' becomes 'here' (here means in this place).
'co' put next to 'here' is 'COHERE'.
Can you help me to learn more?
(Other definitions for cohere that I've seen before include "Cling" , "Hold firmly together" , "Stick together, remain united" , "Form a whole" , "Fuse together" .)