Gourmet to worry monarch (5)
I believe the answer is:
eater
'gourmet' is the definition.
(thesaurus)
'worry monarch' is the wordplay.
'worry' becomes 'eat' (something worrying may eat someone up).
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina).
'eat'+'er'='EATER'
'to' is the link.
(Other definitions for eater that I've seen before include "type of fruit" , "Person taking course" , "Consumer at table" , "Consumer of food" , "Fruit (but not a cooker)" .)