Abandoned by mother, his partner would remain (5)
I believe the answer is:
magog
'his partner would remain' is the definition.
The definition suggests a singular noun which matches the answer.
'abandoned by mother' is the wordplay.
'abandoned' becomes 'gog' (I am not sure about this - if you are sure you should believe this answer much more).
'by' means one lot of letters go next to another.
'mother' becomes 'ma' (ma can mean mother).
'gog' put after 'ma' is 'MAGOG'.
Can you help me to learn more?
(Other definitions for magog that I've seen before include "Last of two old giants" , "Gog and -, City of London guardian giants" , "Biblical king" , "Gog's companion" , "old giant" .)