Half-life close to being straight (6)
I believe the answer is:
linear
'being straight' is the definition.
(I've seen this in another clue)
'half-life close' is the wordplay.
'half life' becomes 'li' (half of word 'life').
'close' becomes 'near' (synonyms).
'li'+'near'='LINEAR'
'to' is the link.
(Other definitions for linear that I've seen before include "In one dimension, I learn (anag.)" , "following straight path" , "Arranged in straight fashion" , "adopting common narrative style" , "Measured lengthwise" .)