Heartlessly delivered a letter from Thessaloniki (5)
I believe the answer is:
delta
'letter from thessaloniki' is the definition.
I can't judge whether this defines the answer.
'heartlessly delivered a' is the wordplay.
'heartlessly' means to remove the middle letters.
'delivered' becomes 'dealt' (I've seen this before).
'dealt' with its centre removed is 'delt'.
'delt'+'a'='DELTA'
Can you help me to learn more?
(Other definitions for delta that I've seen before include "Dealt (anag.)" , "Triangular river system" , "Estuary area where river has split" , "Comms code for 'd'" , "Fourth Greek letter? - area at river mouth" .)