A question on a parasite mostly living in water (7)
I believe the answer is:
aquatic
'living in water' is the definition.
(I've seen this in another clue)
'a question on a parasite mostly' is the wordplay.
'question' becomes 'qu' (abbreviation).
'on' is a charade indicator (letters next to each other).
'parasite' becomes 'tick' (type of parasite).
'mostly' means to remove the last letter (most but not all of the word).
'tick' with its last letter removed is 'tic'.
'a'+'qu'+'a'+'tic'='AQUATIC'
(Other definitions for aquatic that I've seen before include "Living in or near water" , "Relating to water" , "Linked to water" , "At home in water" , "Growing or living in water" .)