The infection I had was watery (5)
I believe the answer is:
fluid
'watery' is the definition.
(thesaurus)
'the infection i had' is the wordplay.
'the infection' becomes 'flu' (I've seen this before).
'i had' becomes 'id' (I'd).
'flu'+'id'='FLUID'
'was' is the link.
(Other definitions for fluid that I've seen before include "Liquid or gas" , "Not solid" , "Aqueous" , "Flowing" .)