At first really envious, eying fine part of sail (4)
I believe the answer is:
reef
'sail' is the definition.
(thesaurus)
'at first really envious eying fine part' is the wordplay.
'at' says to put letters next to each other.
'first really' becomes 'r' (1st letter of 'really').
'part' suggests taking the first letters.
The initial letters of 'envious eying fine' is 'eef'.
'r' next to 'eef' is 'REEF'.
'of' is the link.
(Other definitions for reef that I've seen before include "Coral-ridge" , "Reduce a sail's surface - shipping hazard" , "Undersea ridge; reduce sail area" , "Rock near water's surface" , "Gather up" .)