Half of them will have trouble getting message (5)
I believe the answer is:
'message' is the definition.
(I've seen this before)
'half of them will have trouble' is the wordplay.
'half of' suggests halving the letters.
'them' becomes 'em'.
'will have' becomes 'm' (I am not sure about this - if you are sure you should believe this answer much more).
'trouble' becomes 'ail' (synonyms).
'em' with half the letters taken is 'e'.
'e'+'m'+'ail'='EMAIL'
'getting' acts as a link.
Can you help me to learn more?
(Other definitions for email that I've seen before include "Spam perhaps" , "Messages via the internet" , "Internet-borne communication" , "Message sent via computer" , "Electronic message" .)