Companionable chap, a Republican, promises to pay (10)
I believe the answer is:
gregarious
'companionable' is the definition.
(thesaurus)
'chap a republican promises to pay' is the wordplay.
'chap' becomes 'Greg' (man's name).
'republican' becomes 'R' (political abbreviation).
'promises to pay' becomes 'IOU's' (promises to pay a debt).
'greg'+'a'+'r'+'ious'='GREGARIOUS'
(Other definitions for gregarious that I've seen before include "Enjoying the company of others" , "Living in flocks" , "Sociable" , "Outgoing" , "tending to flock" .)