Work found regularly in two mills (4)
I believe the answer is:
toil
'work' is the definition.
(toil is a kind of work)
'regularly in two mills' is the wordplay.
'regularly in' means one should take alternating letters.
'two'+'mills'='twomills'
The alternating letters of 'twomills' are 'TOIL'.
'found' is the link.
(Other definitions for toil that I've seen before include "Drudgery" , "Work hard, labour" , "Manual labour" , "Work incessantly" , "Work really hard" .)