Ale without a head? That is unnatural (5)
I believe the answer is:
eerie
'unnatural' is the definition.
('eerie' can be similar in meaning to 'unnatural')
'ale without a head? that is' is the wordplay.
'ale' becomes 'beer' (I've seen this before).
'without a head?' means to remove the first letter (I've seen 'without a head' mean this).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'beer' with its first letter removed is 'eer'.
'eer'+'ie'='EERIE'
(Other definitions for eerie that I've seen before include "Unnerving" , "Spooky, macabre" , "Strange and disturbing" , "Frighteningly odd" , "Strange, unusual" .)