Liquid carrier's first thing needed for big run (7)
I believe the answer is:
bladder
'liquid carrier's first thing needed' is the definition.
I can't tell whether this defines the answer.
'big run' is the wordplay.
'big' becomes 'b' (I can't justify this - if you can you should believe this answer much more).
'run' becomes 'ladder' (both can mean a rip in tights).
'b'+'ladder'='BLADDER'
'for' is the link.
Can you help me to learn more?
(Other definitions for bladder that I've seen before include "Bag in the lower abdomen" , "Distensible sac" , "Chamber" , "organ" , "may be subject to inflation?" .)