Excited, since heading for Greece (4)
I believe the answer is:
agog
'excited' is the definition.
(similar in meaning)
'since heading for greece' is the wordplay.
'since' becomes 'ago' (I've seen this before).
'heading for' suggests taking the first letters.
The initial letter of 'greece' is 'g'.
'ago'+'g'='AGOG'
(Other definitions for agog that I've seen before include "Unable to wait" , "Wide-eyed and surprised" , "Open-mouthed" , "Eager or curious" , "Very far from indifferent" .)