Phrase not initially a pained expression? (4)
I believe the answer is:
ouch
'a pained expression?' is the definition.
(I've seen this before)
'phrase not initially' is the wordplay.
'phrase' becomes 'couch' (couching is a kind of phrasing).
'not initially' means to remove the first letter.
'couch' with its initial letter taken away is 'OUCH'.
(Other definitions for ouch that I've seen before include "sounds painful" , "response to a bad pun" , "Ow!" , "Expression of pain" , "Reaction when pricked" .)