Wrongly fears losing a vassal (4)
I believe the answer is:
serf
'vassal' is the definition.
(I've seen this in another clue)
'wrongly fears losing a' is the wordplay.
'wrongly' indicates an anagram (I've seen this in other clues).
'losing' suggests deleting specific letters.
'fears' with 'a' taken away is 'fers'.
'fers' is an anagram of 'SERF'.
(Other definitions for serf that I've seen before include "Subservient labourer" , "Peasant, agricultural labourer" , "Tied estate worker" , "Thrall" , "Medieval villein" .)