Many absorbed in conscious thought (9)
I believe the answer is:
sentiment
'thought' is the definition.
(synonyms)
'many absorbed in conscious' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'absorbed in' indicates putting letters inside.
'conscious' becomes 'sentient' (similar in meaning).
'm' inserted inside 'sentient' is 'SENTIMENT'.
(Other definitions for sentiment that I've seen before include "Slush" , "Test men in, say, emotion" , "Emotional thought" , "judgement" , "General feeling" .)