Bra is so tight, causing pressure lines (7)
I believe the answer is:
isobars
'pressure lines' is the definition.
(I've seen this in another clue)
'bra is so tight' is the wordplay.
'tight' is an anagram indicator (tight can mean drunk).
'bra'+'is'+'so'='braisso'
'braisso' with letters rearranged gives 'ISOBARS'.
'causing' is the link.
(Other definitions for isobars that I've seen before include "15 is strong when they are close" , "Lines of equal atmospheric pressure" , "What may be featured on map" , "Lines on map indicating atmospheric pressure" , "Air boss (anag.)" .)