Instruct boy about old passengers on a bus? (9)
I believe the answer is:
coachload
'passengers on a bus?' is the definition.
'instruct boy about old' is the wordplay.
'instruct' becomes 'coach' (coaching is a kind of instructing).
'boy' becomes 'lad' (lad is a kind of boy**).
'about' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'lad' going around 'o' is 'load'.
'coach'+'load'='COACHLOAD'
(Other definitions for coachload that I've seen before include "Group travelling" , "A lot of passengers" .)