Beer firm evacuated Southport plant (7)
I believe the answer is:
alecost
'plant' is the definition.
(alecost is a kind of plant)
'beer firm evacuated southport' is the wordplay.
'beer' becomes 'ale' (I've seen this before).
'firm' becomes 'co' (a firm is a company).
'evacuated' says to hollow out the word (remove centre letters).
'southport' with its middle taken out is 'st'.
'ale'+'co'+'st'='ALECOST'
(Other definitions for alecost that I've seen before include "used to provide flavouring for beer" , "plant" , "Herb" .)