As a stiffener, you need more than one! (4)
I believe the answer is:
bone
'as' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'a stiffener you need more than one' is the wordplay.
I cannot quite see how this works, but
'a' could be 'one' and 'one' is found in the answer.
A single letter 'b' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for bone that I've seen before include "Skeleton component" , "Carpal or tarsal, for example" , "It makes up the skeleton" , "Scapula, radius or tibia?" , "Skeletal segment" .)