European blocks stuff that's the best (5)
I believe the answer is:
cream
'the best' is the definition.
(I've seen this before)
'european blocks stuff' is the wordplay.
'european' becomes 'e' (abbreviation e.g. EU).
'blocks' indicates putting letters inside (the inside letters form a 'blockage').
'stuff' becomes 'cram' (cramming is a kind of stuffing**).
'e' going within 'cram' is 'CREAM'.
'that's' acts as a link.
(Other definitions for cream that I've seen before include "Very best; pale yellow" , "It rises to the top, the best" , "The best part -of the crop" , "An off-white shade" , "Dairy product" .)