Lithium - metal used in jet fighter - is restricting (8)
I believe the answer is:
limiting
'is restricting' is the definition.
(I've seen this before)
'lithium metal used in jet fighter' is the wordplay.
'lithium' becomes 'li' (Li is the chemical symbol for lithium).
'metal' becomes 'tin' (tin is a kind of metal).
'used in' is an insertion indicator.
'jet fighter' becomes 'mig' (I've seen this in other clues).
'tin' inserted into 'mig' is 'miting'.
'li'+'miting'='LIMITING'
(Other definitions for limiting that I've seen before include "Restricting scope or freedom" , "Checking" , "Bounding" , "cutting back" .)