Singer about to break into Welsh number (4)
I believe the answer is:
wren
'singer' is the definition.
(wren is a kind of birdanimal)
'about to break into welsh number' is the wordplay.
'about' becomes 're' (regarding).
'to break into' indicates putting letters inside (inserted letters break into the word).
'welsh' becomes 'W' (abbreviation).
'number' becomes 'n' (n can represent any number).
'w'+'n'='wn'
're' inserted within 'wn' is 'WREN'.
(Other definitions for wren that I've seen before include "Tiny brown songbird with a short upright tail" , "Once a woman in the Royal Navy" , "Man of note" , "Navy girl" , "Farthing's emblem" .)