Mammal in tar pit not completely absorbed (5)
I believe the answer is:
tapir
'mammal' is the definition.
(tapir is a kind of mammal)
'tar pit not completely absorbed' is the wordplay.
'not completely' means to remove the last letter.
'absorbed' indicates putting letters inside.
'pit' with its last letter taken off is 'pi'.
'tar' going around 'pi' is 'TAPIR'.
'in' acts as a link.
(Other definitions for tapir that I've seen before include "Nocturnal mammal with a short flexible proboscis" , "Tropical hoofed mammal with a flexible proboscis" , "I part with the pig-like animal" , "creature" , "nocturnal beast" .)