Firm with British colleague mostly showing rough tactics (8)

'rough tactics' is the definition.
(as in playing hardball)

'firm with british colleague mostly' is the wordplay.
'firm' becomes 'hard' (both can mean durable or strong).
'with' is a charade indicator (letters next to each other).
'british' becomes 'b' (abbreviation e.g. in BBC).
'colleague' becomes 'ally' (ally can mean a colleague or associate).
'mostly' means to remove the last letter (most but not all of the word).
'ally' with its final letter removed is 'all'.
'hard'+'b'+'all'='HARDBALL'

'showing' acts as a link.