Ruler wants stove to keep hot (4)
I believe the answer is:
agha
'ruler' is the definition.
(I've seen this before)
'stove to keep hot' is the wordplay.
'stove' becomes 'aga' (I've seen this before).
'to keep' means one lot of letters goes inside another.
'hot' becomes 'h' (abbreviation eg on taps).
'aga' placed around 'h' is 'AGHA'.
'wants' is the link.
(Other definitions for agha that I've seen before include "Alternative spelling for a Turkish commander" , "Ruler" , "Turkish civil or military leader" , "Turkish officer" .)