A nervous response comes after a question of water (7)
I believe the answer is:
aquatic
'of water' is the definition.
(I've seen this before)
'a nervous response comes after a question' is the wordplay.
'nervous response' becomes 'tic' (a nervous tic).
'comes after' says to put letters next to each other.
'question' becomes 'qu'.
'a'+'tic'='atic'
'a'+'qu'='aqu'
'atic' after 'aqu' is 'AQUATIC'.
(Other definitions for aquatic that I've seen before include "such as teal?" , "Water-living" , "Plant living in water" , "Water-related" , "Relating to water" .)