Lay a rug with regret, on reflection (7)
I believe the answer is:
amateur
'lay' is the definition.
('amateur' can be similar in meaning to 'lay')
'a rug with regret on reflection' is the wordplay.
'rug' becomes 'mat' (I've seen this before).
'with' means one lot of letters go next to another.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'on reflection' says the letters should be written backwards.
'rue' back-to-front is 'eur'.
'a'+'mat'+'eur'='AMATEUR'
(Other definitions for amateur that I've seen before include "unskilful" , "Dilettante: buff" , "Unpaid (sportsman}" , "Unpaid (sports)" , "incompetent" .)