Letter: it's likely to arrive after Thursday (5)
I believe the answer is:
theta
'letter' is the definition.
(Greek letter)
'it's likely to arrive after thursday' is the wordplay.
I cannot really understand how this works, but
'thursday' could be 'th' (common abbreviation) and 'th' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
The remaining letters 'ea' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for theta that I've seen before include "character in Marathon" , "Eighth letter of Greek alphabet" , "Greek letter equivalent to th" , "letter from abroad" , "Greek letter used in trigonometry" .)