Maybe heart of gold with plague over (5)
I believe the answer is:
organ
'maybe heart' is the definition.
(the heart is an example)
'gold with plague over' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'with' says to put letters next to each other.
'plague' becomes 'nag' (both can mean to bother or annoy).
'over' says the letters should be written in reverse.
'nag' written backwards gives 'gan'.
'or'+'gan'='ORGAN'
'of' acts as a link.
(Other definitions for organ that I've seen before include "Agency" , "notes issue from this" , "With prompt enthusiasm" , "with a lofty tone?" , "specialist magazine" .)