It flows, steady at intervals (3)
I believe the answer is:
tay
'it flows' is the definition.
(River Tay)
'steady at intervals' is the wordplay.
'at intervals' means one should take alternating letters (letters are taken at regular intervals).
The alternate letters of 'steady' are 'TAY'.
(Other definitions for tay that I've seen before include "Scottish river in Katayama?" , "Scotland's longest river" , "The - Bridge Disaster (McGonagall)" , "Dundee's river" , "Longest Scottish river" .)