Fitting bolster back on (6)
I believe the answer is:
proper
'fitting' is the definition.
('proper' can be similar in meaning to 'fitting')
'bolster back on' is the wordplay.
'bolster' becomes 'prop'.
'back' says the letters should be written backwards.
'on' becomes 're' ('on' can be mean 'with reference to').
're' backwards is 'er'.
'prop'+'er'='PROPER'
(Other definitions for proper that I've seen before include "Genuine; correct" , "Conforming to etiquette" , "Real; genuine" , "Right, correct or appropriate" , "name someone's given" .)