Backtracking, English are ready to impound small, small pieces (8)
I believe the answer is:
tesserae
'pieces' is the definition.
(I have seen 'Mosaic pieces ' mean 'tesserae' so perhaps 'pieces' could also mean 'tesserae')
'backtracking english are ready to impound small small' is the wordplay.
'backtracking' is a reversal indicator.
'english' becomes 'e' (abbreviation).
'ready to' becomes 'set' (synonyms).
'impound' indicates putting letters inside.
'small small' becomes 's' (abbreviation - e.g. clothes size; 'small' asks for a short form).
'e'+'are'+'set'='eareset'
'eareset' enclosing 's' is 'earesset'.
'earesset' in reverse letter order is 'TESSERAE'.
(Other definitions for tesserae that I've seen before include "various bits of the picture" , "Tiles" , "Mosaic pieces" , "Passwords" .)