Flag-waving threesome in pact, I fancy (9)
I believe the answer is:
patriotic
'flag-waving' is the definition.
('patriotic' can be similar in meaning to 'flag-waving')
'threesome in pact i fancy' is the wordplay.
'threesome' becomes 'trio' ('trio' can be a synonym of 'threesome').
'in' indicates putting letters inside.
'fancy' indicates an anagram.
'pact'+'i'='pacti'
'pacti' anagrammed gives 'patic'.
'trio' inserted into 'patic' is 'PATRIOTIC'.
(Other definitions for patriotic that I've seen before include "Having love for one's country" , "Loyal" , "flag-waving?" , "Supporting the state" , "Devoted to one's country" .)