Mysterious erotic frolicking on the outskirts of Eccles (8)
I believe the answer is:
esoteric
'mysterious' is the definition.
(I've seen this before)
'erotic frolicking on the outskirts of eccles' is the wordplay.
'frolicking' indicates anagramming the letters.
'on' is a charade indicator (letters next to each other).
'the outskirts of' suggests removing the centre.
'eccles' with its centre taken out is 'es'.
'erotic' is an anagram of 'oteric'.
'oteric' after 'es' is 'ESOTERIC'.
(Other definitions for esoteric that I've seen before include "Intelligible only to informed people in coteries" , "Other-worldly" , "'Abstruse, arcane (8)'" , "Cryptic" , "Abstruse; secret" .)