Is firm with first person when they arrive (4)
I believe the answer is:
come
'arrive' is the definition.
(synonyms)
'is firm with first person' is the wordplay.
'is firm' becomes 'co' (a firm is a company).
'with' is a charade indicator (letters next to each other).
'first person' becomes 'me' (first-person pronoun).
'co'+'me'='COME'
'when they' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for come that I've seen before include "Make a forceful impression" , "Arrive" , "See 13 across" , "' - - fly with me'" , "Approach" .)