Prime minister’s dismal horse (4)
I believe the answer is:
grey
I believe this is a triple definition!
'prime minister's' is the first definition.
(I've seen this before)
'dismal' is the second definition.
(thesaurus)
'horse' is the third definition.
(I've seen this before)
(Other definitions for grey that I've seen before include "Aged; cloudy" , "Dismal - colour" , "showing his age?" , "Colour (of fifty shades?)" , "Looking old" .)