Form of life in river covered in flowing liquid? (5)
I believe the answer is:
larva
'form of life' is the definition.
(immature insect)
'river covered in flowing liquid?' is the wordplay.
'river' becomes 'r' (abbreviation).
'covered in' indicates putting letters inside.
'flowing liquid?' becomes 'lava' (liquid from a volcano).
'r' going inside 'lava' is 'LARVA'.
'in' is the link.
(Other definitions for larva that I've seen before include "Grub, caterpillar stage" , "Maggots" , "Grub; insect" , "Tadpole or caterpillar, say" , "nit?" .)