Most importantly, have a ball! Unlimited cover’s included (5,3)
I believe the answer is:
'most importantly' is the definition.
'a ball unlimited cover's included' is the wordplay.
'unlimited' means to look at the middle letters (discard the edges or 'limits' of the word).
'included' means one lot of letters goes inside another.
The central letters of 'cover' are 'ove'.
'ball' enclosing 'ove' is 'boveall'.
'have' acts as a link.
(Other definitions for above all that I've seen before include "Like the managing director" , "Shakespeare: 'This . . . . . . . ., to thine own self be true'" , "at the end of the day?" , "Mainly" , "omnipotent?" .)