Maiden on dash home — that may account for dark countenance (7)
I believe the answer is:
melanin
'that may account for dark countenance' is the definition.
(dark skin pigment)
'maiden on dash home' is the wordplay.
'maiden' becomes 'm' (cricket abbreviation).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'dash' becomes 'elan' (synonyms).
'home' becomes 'in' (to be in is to be (at) home).
'm'+'elan'+'in'='MELANIN'
(Other definitions for melanin that I've seen before include "Pigment causing tanning" , "Light-absorbing skin pigment" , "Tanning pigment in skin" , "tanning agent" , "Pigment responsible for tanning" .)