Service, first of all, wayward: miles away in defeat (3,4)
I believe the answer is:
low mass
'service' is the definition.
(I have seen 'simple service' mean 'low mass' so perhaps 'service' could also mean 'low mass')
'first of all wayward miles away in defeat' is the wordplay.
'first of all' indicates taking the first letters.
'in' is an insertion indicator.
'defeat' becomes 'loss' (I've seen this in another clue).
The initial letters of 'wayward miles away' is 'wma'.
'wma' put inside 'loss' is 'LOW MASS'.
(Other definitions for low mass that I've seen before include "Simple RC service" , "What a lightweight has" .)