Matter for regret in mine, last of many (4)
I believe the answer is:
pity
'matter for regret' is the definition.
(I've seen this before)
'mine last of many' is the wordplay.
'mine' becomes 'pit' (down the pit**).
'last of' says to take the final letters.
The last letter of 'many' is 'y'.
'pit'+'y'='PITY'
'in' acts as a link.
(Other definitions for pity that I've seen before include "true compassion?" , "Feeling of sorrow for another's pain" , "Sorrow, compassion" , "Feel compassion for" , "Sympathy and sorrow" .)