Frank made nieces accommodate Rex (7)
I believe the answer is:
sincere
'frank' is the definition.
('sincere' can be similar in meaning to 'frank')
'made nieces accommodate rex' is the wordplay.
'made' is an anagram indicator.
'accommodate' indicates putting letters inside.
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'nieces' anagrammed gives 'sincee'.
'sincee' enclosing 'r' is 'SINCERE'.
(Other definitions for sincere that I've seen before include "Free from pretence" , "Candid" , "like informal correspondents?" , "devout" , "Honest, heartfelt" .)