Throw your weight behind B1's strength of character (8)
I believe the answer is:
backbone
'strength of character' is the definition.
(I've seen this before)
'throw your weight behind b1's' is the wordplay.
I cannot quite understand how this works, but
'behind' could be 'back' (similar in meaning) and 'back' is present in the answer.
'b1' could be 'b' and 'b' is present in the answer.
The remaining letters 'one' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for backbone that I've seen before include "Strength of character" , "Spinal column courage" , "Lt protects the spinal cord" , "Spine or courage" , "Firmness of character" .)