Commercial break is lacking purpose (6)
I believe the answer is:
adrift
'lacking purpose' is the definition.
(I've seen this in another clue)
'commercial break' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'break' becomes 'rift' (rift can mean a break or opening).
'ad'+'rift'='ADRIFT'
'is' acts as a link.
(Other definitions for adrift that I've seen before include "Aimlessly floating" , "Wandering aimlessly" , "like Mary Celeste?" , "Floating unattached as at sea" , "No longer fixed in position" .)