Somewhat dark horse (4)
I believe the answer is:
grey
This might be a double definition.
'somewhat dark' is the first definition.
'grey' can be an answer for 'dark' (thesaurus). I am not sure about the 'somewhat' bit.
'horse' is the second definition.
(I've seen this before)
Can you help me to learn more?
(Other definitions for grey that I've seen before include "Neutral tone between black and white" , "Looking old" , "Nondescript; overcast (sky)" , "showing his age?" , "Bushy-tailed North America rodent, now a UK pest" .)