Half-life gets very uniform (6)
I believe the answer is:
livery
'uniform' is the definition.
(livery is a type of formal uniform)
'half-life gets very' is the wordplay.
'half life' becomes 'li' (half of word 'life').
'gets' means one lot of letters go next to another.
'li'+'very'='LIVERY'
(Other definitions for livery that I've seen before include "Uniform, eg of guild" , "Irritable" , "The care of horses for pay" , "Feeding and stabling of horses for money" , "group of servants" .)