Regret going back to work with Earl in a lot of countries (6)
I believe the answer is:
europe
'a lot of countries' is the definition.
(I've seen this before)
'regret going back to work with earl' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'going back' says the letters should be written backwards.
'to work' becomes 'op' (abbreviation for opus).
'with' means one lot of letters go next to another.
'earl' becomes 'e'.
'rue' back-to-front is 'eur'.
'eur'+'op'+'e'='EUROPE'
'in' is the link.
(Other definitions for europe that I've seen before include "Land mass from the Atlantic to the Urals" , "Second smallest continent" , "Second-smallest continent" , "several countries" , "O, puree on the continent" .)