Fellow feeling yen to pursue quarry (4)
I believe the answer is:
pity
'fellow feeling' is the definition.
(pity is a kind of fellow feeling)
'yen to pursue quarry' is the wordplay.
'yen' becomes 'y'.
'to pursue' says to put letters next to each other.
'quarry' becomes 'pit' ('pit' can be a synonym of 'quarry').
'y' put after 'pit' is 'PITY'.
(Other definitions for pity that I've seen before include "Sympathy for the suffering of another" , "Sorrow, compassion" , "Cause for regret" , "Feel sorrow for" , "Mercy" .)