Relevant support in Apple's first operating system (7)
I believe the answer is:
apropos
'relevant' is the definition.
(I've seen this before)
'support in apple's first operating system' is the wordplay.
'support' becomes 'prop' (prop is a kind of support**).
'in' means one lot of letters goes inside another.
'first' says to take the initial letters.
'operating system' becomes 'os' ('OS' can be a synonym of 'operating system').
The first letter of 'apples' is 'a'.
'a'+'os'='aos'
'prop' going inside 'aos' is 'APROPOS'.
(Other definitions for apropos that I've seen before include "Suitable or relevant" , "On the subject of" , "well-chosen" , "Suitable; to the point" , "Concerned with" .)