Cool-headed American male, as a rule (6)
I believe the answer is:
custom
'a rule' is the definition.
(thesaurus)
'cool-headed american male' is the wordplay.
'cool-headed' becomes 'c' (this might be a standard abbreviation of which I'm unaware).
'american' becomes 'us' (abbreviation for United States).
'male' becomes 'tom' (I've seen this before).
'c'+'us'+'tom'='CUSTOM'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for custom that I've seen before include "Habitual action" , "Regular process" , "Established usage" , "Traditional activity or practice" , "Traditional habit" .)