Bringing in rein, nag falters (7)
I believe the answer is:
earning
'bringing in' is the definition.
('earn' can be a synonym of 'bring in')
'rein nag falters' is the wordplay.
'falters' is an anagram indicator (I've seen 'faltering' mean this).
'rein'+'nag'='reinnag'
'reinnag' is an anagram of 'EARNING'.
(Other definitions for earning that I've seen before include "Being paid for work" , "Making money" , "Gaining a wage" , "Getting" , "Inn rage (anag.)" .)