Initially loath, one gathers it's common sense (5)
I believe the answer is:
logic
'sense' is the definition.
(logic is a kind of sense)
'initially loath one gathers it's common' is the wordplay.
'initially' says to take the initial letters.
The initial letters of 'loath one gathers its common' is 'LOGIC'.
(Other definitions for logic that I've seen before include "Good reason" , "The science of reasoning correctly" , "System of reasoning" , "Reasoned thinking" , "Rigorous reasoning" .)