When there's the melee outside, hurry (5)
I believe the answer is:
haste
'hurry' is the definition.
(haste is hurrying or rushing)
'when there's the melee outside' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'there's' means one lot of letters go next to another.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'melee' becomes 'he' (I am not sure about this - if you are sure you should believe this answer much more).
'outside' means one lot of letters goes inside another.
'as'+'t'='ast'
'ast' put inside 'he' is 'HASTE'.
Can you help me to learn more?
(Other definitions for haste that I've seen before include "Excessive speed or urgency" , "Over-eager speed" , "''More . . . . ., less speed'' (5)" , "''Marry in . . . . ., repent at leisure''" , "Rush; urgency" .)