Motor sport competitor’s sidekick lives in outskirts of Rabat (8)
I believe the answer is:
rallyist
'motor sport competitor's' is the definition.
(participant in a rally race)
'sidekick lives in outskirts of rabat' is the wordplay.
'sidekick' becomes 'ally' (a sidekick is a friend or ally).
'lives' becomes 'is' ('be' can be a synonym of 'live').
'in' is an insertion indicator.
'outskirts of' suggests removing the centre (only letters on the outskirts).
'rabat' with its centre taken out is 'rt'.
'ally'+'is'='allyis'
'allyis' put within 'rt' is 'RALLYIST'.
(Another definition for rallyist that I've seen is " Competitive driver".)