Endlessly fooling around in fun, I admit (2,9)
I believe the answer is:
ad infinitum
'endlessly' is the definition.
(I've seen this before)
'fooling around in fun i admit' is the wordplay.
'fooling around' is an anagram indicator.
'in'+'fun'+'i'+'admit'='infuniadmit'
'infuniadmit' is an anagram of 'AD INFINITUM'.
(Other definitions for ad infinitum that I've seen before include "Endlessly, forever (Lat.)" , "Indefinitely" .)