Unrestrained, extremely eager, singular creatures (6)
I believe the answer is:
otters
'creatures' is the definition.
'unrestrained extremely eager singular' is the wordplay.
'unrestrained' becomes 'ott' (abbreviation for 'over the top').
'extremely' means to remove the middle letters.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'eager' with its centre taken out is 'er'.
'ott'+'er'+'s'='OTTERS'
(Other definitions for otters that I've seen before include "Water animals with webbed feet and flat tails" , "Freshwater carnivorous mammals" , "Sleek short-furred swimmers" , "Aquatic carnivores" , "Mammals with webbed feet" .)