To attend carefully, inquiring trusted sources? Understood (5)
I believe the answer is:
tacit
'understood' is the definition.
(I've seen this before)
'to attend carefully inquiring trusted sources?' is the wordplay.
'sources?' indicates taking the first letters (I've seen 'source of' mean this).
The first letters of 'to attend carefully inquiring trusted' is 'TACIT'.
(Other definitions for tacit that I've seen before include "Implied or understood, not put into words" , "Silently understood" , "Aircraft engine casing" , "Silent - understood" , "Implied without being said" .)