Minimum piece, simple as that (5)
I believe the answer is:
least
'minimum' is the definition.
('least' can be similar in meaning to 'minimum')
'piece simple as that' is the wordplay.
'piece' indicates a hidden word.
'LEAST' is hidden within 'simple as that'.
(Other definitions for least that I've seen before include "No more petty" , "Smallest in extent" , "Minimal amount" , "Superlative form of 'little'" , "superlatively small" .)