Fish seen regularly in Trent area (5)
I believe the answer is:
tetra
'fish' is the definition.
(tetra is a kind of fish)
'regularly in trent area' is the wordplay.
'regularly in' indicates alternate letters.
'trent'+'area'='trentarea'
The alternating letters of 'trentarea' are 'TETRA'.
'seen' is the link.
(Other definitions for tetra that I've seen before include "in groups of four" , "Bright tropical fish" , "This indicates a group of four" , "Bright fish; four (prefix)" , "Denoting four (prefix)" .)