Close tightly; mammal (4)
I believe the answer is:
seal
This clue is a double definition.
'close tightly' is the first definition.
(I've seen this in another clue)
'mammal' is the second definition.
(type of mammal)
(Other definitions for seal that I've seen before include "Stamp of approval" , "marine creature" , "Marine mammal for sale" , "Signet" , "A mammal; one that secures" .)