Difference in years, for example, between a girl's first and a chap's last (3,3)
I believe the answer is:
age gap
'difference in years' is the definition.
(I've seen this before)
'example between a girl's first and a chap's last' is the wordplay.
'example' becomes 'eg'.
'between' indicates putting letters inside.
'a girl' becomes 'ag' (**).
'first' says to take the initial letters.
'last' suggests the final letters.
The initial letter of 'and' is 'a'.
The final letter of 'chap' is 'p'.
'ag'+'a'='aga'
'eg' inserted inside 'aga' is 'agega'.
'agega'+'p'='AGE GAP'
'for' is the link.
(Other definitions for age gap that I've seen before include "Difference between generations" , "Difference in years" .)