Reason one joins record clubs (5)
I believe the answer is:
'reason' is the definition.
(I've seen this before)
'one joins record clubs' is the wordplay.
'one' becomes 'i' (Roman numeral).
'joins' means one lot of letters go next to another.
'record' becomes 'log' (logging is a kind of recording).
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'i' put after 'log' is 'logi'.
(Other definitions for logic that I've seen before include "using premises" , "this makes sense" , "Reasoned argument" , "Branch of philosophy that analyses inference" , "4-ing" .)