Many let it damage intellectual power (9)
I believe the answer is:
mentality
'intellectual power' is the definition.
(thesaurus)
'many let it damage' is the wordplay.
'damage' is an anagram indicator (I've seen 'damaged' mean this).
'many'+'let'+'it'='manyletit'
'manyletit' anagrammed gives 'MENTALITY'.
(Other definitions for mentality that I've seen before include "See 7" , "Way of thinking" .)