Beer with no head: that is weird (5)
I believe the answer is:
eerie
'weird' is the definition.
(both can mean strange or unsettling)
'beer with no head that is' is the wordplay.
'with no head' means to remove the first letter.
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'beer' with its first letter taken off is 'eer'.
'eer'+'ie'='EERIE'
(Other definitions for eerie that I've seen before include "Spooky, unearthly" , "Awesome" , "Strangely frightening" , "Scary, weird" , "Mysterious" .)