Yell 'Get away!' with first sight of enemy entering (6)
I believe the answer is:
scream
'yell' is the definition.
(synonyms)
'away with first sight of enemy entering' is the wordplay.
I cannot really see how this works, but
'away' could be 'a' (abbreviation as used in sports fixtures) and 'a' is present in the answer.
'entering' could be 're' (I've seen this in other clues) and 're' is found within the answer.
'first sight' could be 's' (1st letter of 'sight') and 's' is found in the answer.
This explanation may well be incorrect...
'get' acts as a link.
(Other definitions for scream that I've seen before include "'Sharp, piercing cry (6)'" , "immortalised by Edvard Munch?" , "Utter a piercing cry" , "Piercing yell" , "Hilarious character" .)