Express surprise at half of them and interject (4)
I believe the answer is:
ahem
'express surprise at half of them and interject' is the definition.
I can't judge whether this defines the answer.
'at half of them and interject' is the wordplay.
'half of' means to take half the letters.
'them' becomes 'em' ('em is an informal way of saying 'them').
'and' says to put letters next to each other.
'interject' becomes 'h' (I am not sure about this - if you are sure you should believe this answer much more).
'at' with half the letters taken is 'a'.
'em' put after 'h' is 'hem'.
'a'+'hem'='AHEM'
Can you help me to learn more?
(Other definitions for ahem that I've seen before include "Throat clearing sound to catch attention" , "It's said to gain attention; ah, me (anag.)" , "Cough to draw attention" , "Gentle utterance to attract attention" , "Attention-getter" .)