Meadowlands contributing to unalloyed contentment (8)
I believe the answer is:
pleasure
'contentment' is the definition.
(I've seen this before)
'meadowlands contributing to unalloyed' is the wordplay.
'meadowlands' becomes 'leas' (I've seen this in another clue).
'contributing to' indicates putting letters inside (inserted letters make a new contribution).
'unalloyed' becomes 'pure' (similar in meaning).
'leas' inserted within 'pure' is 'PLEASURE'.
(Other definitions for pleasure that I've seen before include "entertainment" , "A desirable emotion" , "diversion" , "'Enjoyment, delight (8)'" , "Delectation, delight" .)