Ram one beset by twitching ears (5)
I believe the answer is:
aries
'ram' is the definition.
(I've seen this in another clue)
'one beset by twitching ears' is the wordplay.
'one' becomes 'i' (Roman numeral).
'beset by' means one lot of letters goes inside another ('beset' could historically mean 'cover').
'twitching' indicates an anagram (letters twitch or move into a new order).
'ears' with letters rearranged gives 'ares'.
'i' put within 'ares' is 'ARIES'.
(Other definitions for aries that I've seen before include "First sign of the zodiac" , "A sign from Heaven" , "Constellation with sign of the ram" , "Ram star sign" , "Zodiacal sign also known as the Ram" .)