Endless equations originally learnt badly (7)
I believe the answer is:
eternal
'endless' is the definition.
(I've seen this before)
'equations originally learnt badly' is the wordplay.
'originally' says to take the initial letters.
'badly' indicates an anagram (letters in the wrong order).
The initial letter of 'equations' is 'e'.
'learnt' is an anagram of 'ternal'.
'e'+'ternal'='ETERNAL'
(Other definitions for eternal that I've seen before include "Valid for all time" , "Infinite - endless" , "going on for ever" , "Continuing forever or indefinitely" , "Seemingly without end" .)