Thought many had been exploited (5)
I believe the answer is:
mused
'thought' is the definition.
(musing is a kind of thinking)
'many had been exploited' is the wordplay.
'many' becomes 'M' (Roman numeral for a thousand).
'had' is a charade indicator (letters next to each other).
'been exploited' becomes 'used' (I can't justify this - if you can you should give a lot more credence to this answer).
'm'+'used'='MUSED'
Can you help me to learn more?
(Other definitions for mused that I've seen before include "'Meditated, mulled (5)'" , "Reflected or pondered" , "Was deep in thought" , "Was absorbed in thought" , "Contemplated" .)