Infinite idleness — one left shaken (7)
I believe the answer is:
'infinite' is the definition.
('endless' can be similar in meaning to 'infinite')
'idleness one left shaken' is the wordplay.
'one' becomes 'i' (Roman numeral).
'left' is a deletion indicator.
'shaken' indicates an anagram.
'idleness' with 'i' taken away is 'dleness'.
'dleness' anagrammed gives 'ENDLESS'.
(Other definitions for endless that I've seen before include "Non-stop" , "Tiresomely long" , "Eternal - unbroken" , "Continuing too long" , "'Eternal, without finish (7)'" .)