Replace small and out of bed shrub, say (8)
I believe the answer is:
supplant
'replace' is the definition.
('supplant' can be a synonym of 'replace')
'small and out of bed shrub say' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' is a charade indicator (letters next to each other).
'out of bed' becomes 'up' ('up' can be a synonym of 'out of bed').
'shrub say' becomes 'plant' (shrub is a kind of plant).
's'+'up'+'plant'='SUPPLANT'
(Other definitions for supplant that I've seen before include "Oust" , "Displace" , "take another's place" , "Supersede and replace" .)