Was happy with new pen? (4)
'pen?' is the definition.
(pen is a female swan)
'was happy with new' is the wordplay.
'happy' indicates an anagram (happy can mean slightly drunk).
'with' means one lot of letters go next to another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'was' anagrammed gives 'swa'.
(Other definitions for swan that I've seen before include "Whooper, perhaps", "Shakespeare is called The .... of Avon", "Whooper or trumpeter?", "Flighty", "Graceful bird".)