Fish put back in some water again (5)
I believe the answer is:
tetra
'fish' is the definition.
(tetra is a kind of fish)
'back in some water again' is the wordplay.
'back' says the letters should be written backwards.
'in' indicates putting letters inside.
'some' means to look at the middle letters.
'water' becomes 'bath' (bath water).
'again' becomes 'rte' (I am not sure about this - if you are sure you should believe this answer much more).
The centre of 'bath' is 'at'.
'at' placed around 'rte' is 'artet'.
'artet' in reverse letter order is 'TETRA'.
'put' is the link.
Can you help me to learn more?
(Other definitions for tetra that I've seen before include "Swimmer" , "Wrongful act (Law)" , "in groups of four" , "Small brightly-coloured fish of aquariums" , "With four parts" .)