Sovereign is backing father (4)
I believe the answer is:
sire
'father' is the definition.
(I've seen this before)
'sovereign is backing' is the wordplay.
'sovereign' becomes 'er' (Abbreviation for Elizabeth Regina).
'backing' shows that the letters should be reversed in order.
'er'+'is'='eris'
'eris' written backwards gives 'SIRE'.
(Other definitions for sire that I've seen before include "Male parent of a horse, for example" , "Stallion; beget" , "Generator" , "Foal's father" , "Make children" .)