Thought of a US soldier wearing denim pants (8)
I believe the answer is:
'thought' is the definition.
('imagine' can be a synonym of 'think')
'a us soldier wearing denim pants' is the wordplay.
'us soldier' becomes 'gi' (I've seen this in other clues).
'wearing' is an insertion indicator.
'pants' indicates anagramming the letters (UK informal term for 'nonsense' or 'rubbish').
'denim' with letters rearranged gives 'imned'.
'agi' placed within 'imned' is 'IMAGINED'.
'of' is the link.
(Other definitions for imagined that I've seen before include "Created mentally" , "Fancied" , "Conjured up in the mind" , "Supposed" , "Formed a mental picture" .)