Swan below weir’s spray (6)
I believe the answer is:
dampen
'spray' is the definition.
(thesaurus)
'swan below weir's' is the wordplay.
'swan' becomes 'pen' (pen is a kind of swan).
'below' says to put letters next to each other (in a down clue, some letters go below others).
'weir' becomes 'dam' (I've seen this in another clue).
'pen' after 'dam' is 'DAMPEN'.
(Other definitions for dampen that I've seen before include "Make moist or subdued" , "Apply liquid!" , "Make slightly moist, or discourage" , "Moisten" , "Make moist or depress spirit" .)