Heard Queen's restricted by unusual diet (5)
I believe the answer is:
tried
'heard' is the definition.
('try' can be a synonym of 'hear')
'queen's restricted by unusual diet' is the wordplay.
'queen' becomes 'r' (r is abbreviation regina. e.g. in ER ).
'restricted by' is an insertion indicator (I've seen this in other clues).
'unusual' indicates an anagram.
'diet' is an anagram of 'tied'.
'r' put into 'tied' is 'TRIED'.
(Other definitions for tried that I've seen before include "Essayed" , "Sampled" , "Made an attempt" , "Tested" , "checked out" .)