Cranium excavated, strangely grey — foul play? (12)
I believe the answer is:
skullduggery
'foul play?' is the definition.
(both can illegal or illicit activity)
'cranium excavated strangely grey' is the wordplay.
'cranium' becomes 'skull' (I've seen this before).
'excavated' becomes 'dug' ('dig' can be a synonym of 'excavate').
'strangely' indicates an anagram.
'grey' anagrammed gives 'gery'.
'skull'+'dug'+'gery'='SKULLDUGGERY'
(Other definitions for skullduggery that I've seen before include "Underhand trickery" , "Underhand behaviour (mainly US)" .)