Mares regularly taking fruit (6)
I believe the answer is:
damson
'fruit' is the definition.
(damson is a kind of fruit)
'mares regularly taking' is the wordplay.
'mares' becomes 'dams' (I can't explain this - if you can you should give a lot more credence to this answer).
'regularly taking' becomes 'on' (as in 'on drugs').
'dams'+'on'='DAMSON'
Can you help me to learn more?
(Other definitions for damson that I've seen before include "Small dark-purple plum" , "Small purple plum" , "jam producer" , "Small plum-like fruit" , "Kind of plum for nomads" .)