Card put back inside uppermost part of boot (6)
I believe the answer is:
toecap
'part of boot' is the definition.
(I know this)
'card put back inside uppermost' is the wordplay.
'card' becomes 'ace' (ace is a type of playing card).
'put back' is a reversal indicator.
'inside' is an insertion indicator.
'uppermost' becomes 'top' (similar in meaning).
'ace' back-to-front is 'eca'.
'eca' inserted inside 'top' is 'TOECAP'.
(Other definitions for toecap that I've seen before include "Strengthener on front of boot, shoe" , "Tip of shoe" , "Boot reinforcement" , "part of Oxford, perhaps" , "Front cover of shoe" .)