Firm initially in short discussion about union (6)

'union' is the definition.
Although both the answer and definition are adjectives, I cannot see how one could define the other.

'firm initially in short discussion' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'initially' becomes 'i' (this could be a standard abbreviation I've not previously seen).
'in short' means to remove the last letter.
'discussion' becomes 'talk' (talk is a kind of discussion).
'talk' with its last letter removed is 'tal'.
'co'+'i'+'tal'='COITAL'

'about' acts as a link.

Can you help me to learn more?