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' is an insertion indicator (inserted letters break into the word).
'welsh' becomes 'W' (abbreviation).
'number' becomes 'n' (n can represent any number).
'w'+'n'='wn'
're' placed inside '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" .)