New T-shirt is long (6)
I believe the answer is:
thirst
'long' is the definition.
(thirst can mean to long or yearn for something)
'new t-shirt' is the wordplay.
'new' indicates an anagram.
't'+'shirt'='tshirt'
'tshirt' with letters rearranged gives 'THIRST'.
'is' is the link.
(Other definitions for thirst that I've seen before include "Strong desire for something" , "Craving for liquid" , "Strong desire for drink" , "Long" , "Strong desire from dehydration?" .)