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.
I cannot quite understand how this works, but
'many' could be 'm' (Roman numeral for a thousand) and 'm' is found within the answer.
'had' could be 'd' (I've seen this in other clues) and 'd' is found in the answer.
The remaining letters 'use' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
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" .)