Thought of a US soldier wearing denim pants (8)
I believe the answer is:
Here is my best explanation:
'thought' is the definition.
('imagine' can be a synonym of 'think')
'a us soldier wearing denim pants' is the subsidiary indication.
'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' is an anagram of 'imned'.
'agi' placed into '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", "Not based on fact, dubious", "Conceived of", "Created in the mind", "Envisaged", "Dreamed" and "Formed a mental image".)