That tiny clue 'informer' is one of those little traps that makes me grin and groan at the same time.
My default fill for short crosswords is usually 'rat'—three letters, blunt, and used so often it's almost crossword shorthand. If the grid wants four letters, I lean toward 'fink' or 'tatt' (if it's playful), and for longer slots 'snitch' (6), 'informant' (8), or 'stoolie' (7) often fit. If the puzzle's from the UK, don't forget 'grass' or 'nark'—British setters love those. For cryptic puzzles the setter might be more sly: 'sings' could be clued by 'informer' (as in 'to sing' = to inform), so verb forms can appear rather than nouns.
Crosses are king. A lot of solvers panic at the clue but once a couple of crossing letters are in, the right fill jumps out. I like how this clue showcases regional vocabulary and setter style; it keeps every puzzle fresh and a little cheeky.