Empty lemonade can designed as a weapon (5)
I believe the answer is:
lance
'a weapon' is the definition.
(I know that lance is a type of weapon)
'empty lemonade can designed' is the wordplay.
'empty' suggests removing the centre.
'designed' is an anagram indicator (designed with the letters).
'lemonade' with its centre taken out is 'le'.
'le'+'can'='lecan'
'lecan' anagrammed gives 'LANCE'.
'as' is the link.
(Other definitions for lance that I've seen before include "Incise" , "Horseman's long weapon" , "corporal may have at the front" , "Jousting weapon" , "Weapon of knights of old" .)