What might stretch boot? Here toes will need flexing (8)
I believe the answer is:
shoetree
'what might stretch boot?' is the definition.
(a shoetree keeps a shoe's shape)
'here toes will need flexing' is the wordplay.
'will need flexing' indicates an anagram (letters move or flex around).
'here'+'toes'='heretoes'
'heretoes' with letters rearranged gives 'SHOETREE'.
(Other definitions for shoetree that I've seen before include "Device put into footwear to maintain shape" , "what keeps loafer in shape" , "It preserves the shape of footwear" , "what's found in Oxford?" , "A shaped block for holding footwear" .)