I err badly, fooling around for a lark? (5,4)
I believe the answer is:
early bird
'a lark?' is the definition.
(I've seen this before)
'i err badly fooling around' is the wordplay.
'fooling around' is an anagram indicator.
'i'+'err'+'badly'='ierrbadly'
'ierrbadly' with letters rearranged gives 'EARLY BIRD'.
'for' acts as a link.
(Other definitions for early bird that I've seen before include "This one catches the worm" , "one up at dawn" , "Vermicidal" , "first to start in the morning" , "Worm catcher?" .)