Clubs then set up for speaking informally (3)
I believe the answer is:
cos
'for speaking informally' is the definition.
I can't judge whether this definition defines the answer.
'clubs then set up' is the wordplay.
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'then' becomes 'so' (both can mean 'therefore' or 'in that case').
'set up' is a reversal indicator (in a down clue, letters go up).
'so' back-to-front is 'os'.
'c'+'os'='COS'
Can you help me to learn more?
(Other definitions for cos that I've seen before include "foreign island" , "Lettuce with long crisp leaves" , "Mozart fidelity-test opera" , "Islanders here" , "vegetable?" .)