Exercising, Lily and I groan to begin with (10)
I believe the answer is:
originally
'to begin with' is the definition.
(I've seen this before)
'exercising lily and i groan' is the wordplay.
'exercising' is an anagram indicator (letters exercise or move around).
'and' says to put letters next to each other.
'i'+'groan'='igroan'
'lily' after 'igroan' is 'igroanlily'.
'igroanlily' is an anagram of 'ORIGINALLY'.
(Other definitions for originally that I've seen before include "new way" , "In the first place" , "At the start" , "to begin with" .)