Austere detectives in CIA sent back (7)
I believe the answer is:
ascetic
'austere' is the definition.
('ascetic' can be a synonym of 'austere')
'detectives in cia sent back' is the wordplay.
'detectives' becomes 'tecs' ('tec' can be a synonym of 'detective').
'in' is an insertion indicator.
'sent back' says the letters should be written backwards.
'tecs' placed into 'cia' is 'citecsa'.
'citecsa' written backwards gives 'ASCETIC'.
(Other definitions for ascetic that I've seen before include "One practises self-denial as a spiritual discipline" , "Self-denying, not allowing oneself luxuries" , "Religiously self-disciplined" , "he never imbibes" , "Self-denier" .)