Fish, last of four in a row (5)
I believe the answer is:
sprat
'fish' is the definition.
(sprat is a kind of fish)
'last of four in a row' is the wordplay.
'last of' says to take the final letters.
'in' means one lot of letters goes inside another.
'a row' becomes 'spat' (spat is a kind of row).
The final letter of 'four' is 'r'.
'r' placed into 'spat' is 'SPRAT'.
(Other definitions for sprat that I've seen before include "Small herring-like fish" , "It may catch a mackerel" , "one on a fat-free diet" , "Small fish, sardine, say" , "It's set to catch a mackerel" .)