Son, mine, an exact likeness (4)
I believe the answer is:
spit
'an exact likeness' is the definition.
'spit' can be an answer for 'likeness' (thesaurus). I am unsure of the 'an exact' bit.
'son mine' is the wordplay.
'son' becomes 's'.
'mine' becomes 'pit' (down the pit).
's'+'pit'='SPIT'
Can you help me to learn more?
(Other definitions for spit that I've seen before include "Rod used for roasting meat" , "Be furious" , "double, perhaps" , "A spade's depth, in earth" , "nearly a foot deep" .)