Sovereign added to rotten bid (5)
I believe the answer is:
offer
'bid' is the definition.
('offer' can be a synonym of 'bid')
'sovereign added to rotten' is the wordplay.
'sovereign' becomes 'er' (Abbreviation for Elizabeth Regina).
'added to' means one lot of letters go next to another.
'rotten' becomes 'off' (I've seen this before**).
'er' after 'off' is 'OFFER'.
(Other definitions for offer that I've seen before include "Put forward" , "Tender, volunteer" , "Make available for sale" , "Tender - proposal" , "Tender or propose" .)