Adapt hybrid cactus, by order (8)
I believe the answer is:
accustom
'adapt' is the definition.
(I've seen this before)
'hybrid cactus by order' is the wordplay.
'hybrid' indicates anagramming the letters (a hybrid is a mixture).
'by' says to put letters next to each other.
'order' becomes 'om' (Order of Merit).
'cactus' with letters rearranged gives 'accust'.
'accust'+'om'='ACCUSTOM'
(Other definitions for accustom that I've seen before include "Make (someone) used to (something)" , "Maybe reconcile" , "Inure" , "get used to" , "Familiarise" .)