Experienced a bird circling river, keeping on and on (4,2,2)
I believe the answer is:
hard at it
'on and on' is the definition.
I know nothing about this answer so I can't judge whether this works.
'experienced a bird circling river' is the wordplay.
'experienced' becomes 'had' (have can mean to experience or go through).
'bird' becomes 'tit' (tit is a kind of bird).
'circling' indicates putting letters inside (circling can mean going around or containing).
'river' becomes 'r' (abbreviation for river).
'had'+'a'+'tit'='hadatit'
'hadatit' placed around 'r' is 'HARD AT IT'.
'keeping' acts as a link.
Can you help me to learn more?
(Other definitions for hard at it that I've seen before include "Very busy - that raid (anag)" , "Working very busily" , "Really busy" .)