A chap one takes on - or a very large number (7)
I believe the answer is:
billion
'a very large number' is the definition.
(I've seen this before)
'a chap one takes on' is the wordplay.
'a chap' becomes 'bill' (short for William).
'one' becomes 'i' (Roman numeral).
'takes' says to put letters next to each other.
'bill'+'i'+'on'='BILLION'
'or' is the link.
(Other definitions for billion that I've seen before include "One thousand million" , "A large number" , "One followed by nine zeros" , "1,000,000,000" .)