Very small chap is old American fighter (9)
I believe the answer is:
minuteman
'old american fighter' is the definition.
'very small chap' is the wordplay.
'very small' becomes 'minute' (I've seen this before).
'chap' becomes 'man' (I've seen this before).
'minute'+'man'='MINUTEMAN'
'is' is the link.
(Other definitions for minuteman that I've seen before include "Someone always ready for action" , "Former American militia member" , "early American fighter" , "One ready to serve" , "American revolutionary militia member" .)