Oddly weak, their drink (5)
I believe the answer is:
water
'drink' is the definition.
(thesaurus)
'oddly weak their' is the wordplay.
'oddly' means one should take alternating letters.
'weak'+'their'='weaktheir'
The alternating letters of 'weaktheir' are 'WATER'.
(Other definitions for water that I've seen before include "One of the four classical elements" , "Commonest fluid" , "non-alcoholic drink" , "Essential liquid" , "Lake or river" .)