Understand criminal's straight (8)
I believe the answer is:
construe
'understand' is the definition.
(construing is a kind of understanding)
'criminal's straight' is the wordplay.
'criminal's' becomes 'cons' (abbreviation for 'convict').
'straight' becomes 'true' ('true' can be a synonym of 'straight').
'cons'+'true'='CONSTRUE'
(Other definitions for construe that I've seen before include "Counters so as to interpret or explain" , "Analyse language" , "Understand" , "Make sense of -- trounces (anag)" , "Read - trounces (anag)" .)