Typically, a Girl Guide's first to wear one? (2,7)
I believe the answer is:
on average
'typically' is the definition.
(I've seen this before)
'a girl guide's first to wear one?' is the wordplay.
'girl' becomes 'vera' (**).
'first' says to take the initial letters.
'to wear' is an insertion indicator.
The first letter of 'guides' is 'g'.
'a'+'vera'+'g'='averag'
'averag' inserted into 'one' is 'ON AVERAGE'.
(Other definitions for on average that I've seen before include "Generally" , "typically speaking" , "Usually" .)