Runner perhaps needing to reserve somewhere to sit (7)
I believe the answer is:
beanbag
'somewhere to sit' is the definition.
'runner perhaps needing to reserve' is the wordplay.
'runner perhaps' becomes 'bean'.
'needing' says to put letters next to each other.
'to reserve' becomes 'bag'.
'bean'+'bag'='BEANBAG'
(Other definitions for beanbag that I've seen before include "Small cloth sack thrown in children's games" , "Large cushion filled with granules" , "somewhere comfy to sit" , "support for student?" , "comfortable seat" .)