One fluid initially permeates invalid sump (2,4)
I believe the answer is:
in full
'one fluid initially permeates invalid sump' is the definition.
I can't judge whether this definition defines the answer.
'one fluid initially permeates invalid' is the wordplay.
'one' becomes 'I' (Roman numeral).
'initially' says to take the initial letters.
'permeates' is an insertion indicator (inserted letters permeate or go through others).
'invalid' becomes 'null' (similar in meaning).
The initial letter of 'fluid' is 'f'.
'f' placed into 'null' is 'nfull'.
'i'+'nfull'='IN FULL'
Can you help me to learn more?
(Other definitions for in full that I've seen before include "The entire amount, with nothing omitted" , "Completely" , "Tobacco constituent" .)