Pre-eminent male, remaining, elderly relative (11)
I believe the answer is:
'elderly relative' is the definition.
'grandmother' can be an answer for 'relative' (grandmother is a kind of relative). I'm not certain of the 'elderly' bit.
'pre-eminent male remaining' is the wordplay.
'pre-eminent' becomes 'grand' (I can't justify this - if you can you should believe this answer much more).
'male' becomes 'm' (common abbreviation).
'remaining' becomes 'other' (similar in meaning).
Can you help me to learn more?
(Other definitions for grandmother that I've seen before include "Female ancestor" , "See 22" , "Relation" , "Older relative" , "Mum's mum" .)