Tip top criticism by mouth! (4)
I believe the answer is:
trap
'mouth' is the definition.
(informal term for mouth)
'tip top criticism by' is the wordplay.
'tip' suggests taking the first letters (I've seen 'tip of' mean this).
'criticism' becomes 'rap' (rap is a kind of criticism**).
'by' means one lot of letters go next to another.
The initial letter of 'top' is 't'.
't'+'rap'='TRAP'
(Other definitions for trap that I've seen before include "Bunker; carriage" , "Means of transport - sometimes sprung!" , "Device to catch prey" , "two-wheeler" , "Snare - mouth" .)