One animal in a line, we're told (3)
I believe the answer is:
roe
'one animal' is the definition.
(I've seen this before)
'a line we're told' is the wordplay.
'a line' becomes 'row' (row is a kind of line).
'we're told' shows a homophone (sound like).
'row' is a homophone of 'ROE'.
'in' is the link.
(Other definitions for roe that I've seen before include "Fish eggs; small deer" , "Swift runner" , "eg Caviare" , "Caviar" , "Mass of fish eggs" .)