Review harshly constrains each piece expressing praise (5)
I believe the answer is:
paean
'piece expressing praise' is the definition.
(song or poem praising someone)
'review harshly constrains each' is the wordplay.
'review harshly' becomes 'pan' (to pan is to heavily criticise ).
'constrains' is an insertion indicator.
'each' becomes 'ea'.
'pan' enclosing 'ea' is 'PAEAN'.
(Other definitions for paean that I've seen before include "Hymn of praise" , "Panegyric" , "Triumphal song" , "Formal expression of praise" , "Anthem" .)