Whenever required, individual chap departs carrying diamonds (2,6)
I believe the answer is:
on demand
'whenever required' is the definition.
(I've seen this before)
'individual chap departs carrying diamonds' is the wordplay.
'individual' becomes 'one' (an individual is one person).
'chap' becomes 'man' (chap can mean a man).
'departs' becomes 'd' (abbreviation used in transport schedules).
'carrying' means one lot of letters goes inside another.
'diamonds' becomes 'd' (abbreviation for the card suit diamonds).
'one'+'man'+'d'='onemand'
'onemand' going around 'd' is 'ON DEMAND'.
(Other definitions for on demand that I've seen before include "When asked for" , "As soon as requested" , "Whenever required" , "As soon as required" .)