Alarming appearance of head of Greek army (5)
I believe the answer is:
ghost
'alarming appearance' is the definition.
'ghost' can be an answer for 'appearance' (ghost is a kind of appearance). I am not sure about the 'alarming' bit.
'head of greek army' is the wordplay.
'head of' says to take the initial letters.
'army' becomes 'host' (host is a kind of army).
The initial letter of 'greek' is 'g'.
'g'+'host'='GHOST'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for ghost that I've seen before include "A spirit from the beyond" , "21 [WRAITH]" , "Lay down life for another in this" , "Hamlet's spectral father" , "Spectre, wraith" .)