Face a girl and a boy (4)
I believe the answer is:
dial
'face' is the definition.
(dial is a kind of face)
'a girl and a boy' is the wordplay.
'a girl' becomes 'di' (**).
'and' means one lot of letters go next to another.
'a boy' becomes 'al'.
'di'+'al'='DIAL'
(Other definitions for dial that I've seen before include "Laid up like clock-face" , "Face of a timepiece" , "Face with numbers" , "Coil of yarn" , "Meter face" .)