Incredibly, cap's all made of the same material (5)
I believe the answer is:
solid
'material' is the definition.
(thesaurus)
'incredibly cap's all made of the same' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is located in the answer.
'cap' could be 'lid' (eg the lid of a bottle) and 'lid' is found within the answer.
A single letter 's' 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 solid that I've seen before include "Shape with three dimensions" , "Firm - sound - reliable" , "Hard and compact" , "thick" , "Of substance" .)