Think, love, America is massive (9)
I believe the answer is:
ponderous
'massive' is the definition.
(thesaurus)
'think love america' is the wordplay.
'think' becomes 'ponder' (pondering is a kind of thinking**).
'love' becomes 'o' (In tennis, 'love' means 'zero').
'america' becomes 'us' (abbreviation for the United States).
'ponder'+'o'+'us'='PONDEROUS'
'is' is the link.
(Other definitions for ponderous that I've seen before include "Slow and laborious" , "Weighty" , "Lumbering" , "Heavy and slow-moving" , "Heavy-handed" .)