Amiable clot keeps backing in (9)
I believe the answer is:
congenial
'amiable' is the definition.
(thesaurus)
'clot keeps backing in' is the wordplay.
'clot' becomes 'congeal' (I've seen this before**).
'keeps' means one lot of letters goes inside another.
'backing' is a reversal indicator.
'in' written backwards gives 'ni'.
'congeal' enclosing 'ni' is 'CONGENIAL'.
(Other definitions for congenial that I've seen before include "relaxing" , "Suited to one's taste" , "Having similar interests" , "Harmonious and friendly" , "Pleasant and agreeable" .)