String in wet fluid (5)
I believe the answer is:
twine
'string' is the definition.
(I know that string can be written as twine)
'in wet fluid' is the wordplay.
'fluid' indicates an anagram.
'in'+'wet'='inwet'
'inwet' with letters rearranged gives 'TWINE'.
(Other definitions for twine that I've seen before include "Curl; string" , "wind" , "twist" , "String of twisted fibres" , "Stout string" .)