Creatures from Greenland ultimately, always under control (8)
I believe the answer is:
reindeer
'creatures' is the definition.
(type of animal)
'greenland ultimately always under control' is the wordplay.
'ultimately' says to take the final letters.
'always' becomes 'eer' (poetic way of saying ever).
'under' means one lot of letters go next to another.
'control' becomes 'rein' (reining is a kind of controlling).
The final letter of 'greenland' is 'd'.
'd'+'eer'='deer'
'deer' put after 'rein' is 'REINDEER'.
'from' is the link.
(Other definitions for reindeer that I've seen before include "They're called caribou in North America" , "Dancer's one" , "They pull Santa's sleigh" , "D [DECORATION]" , "Red-nosed Rudolph is one" .)