Bishop worried about letter to Greek ambassador (4)
I believe the answer is:
beta
'letter to greek ambassador' is the definition.
I can't judge whether this definition defines the answer.
'bishop worried about' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'worried' becomes 'ate' (something worrying may eat someone up).
'about' says the letters should be written backwards.
'ate' written backwards gives 'eta'.
'b'+'eta'='BETA'
Can you help me to learn more?
(Other definitions for beta that I've seen before include "testing stage" , "Second Greek letter, B" , "Second-rate" , "It's next after alpha" , "Letter before gamma" .)