Frank or poor Dan locked up by police (6)
I believe the answer is:
candid
'frank' is the definition.
(synonyms)
'poor dan locked up by police' is the wordplay.
'poor' indicates anagramming the letters.
'locked up by' indicates putting letters inside.
'police' becomes 'CID' (abbreviation for Criminal Investigation Department).
'dan' is an anagram of 'and'.
'and' put within 'cid' is 'CANDID'.
'or' acts as a link.
(Other definitions for candid that I've seen before include "Blunt; frank" , "Forthrightly sincere" , "Impartial" , "Free and frank" , "Frank, honest" .)