Ballet position of sailor in lively square beginning to evolve (9)
I believe the answer is:
arabesque
'ballet position' is the definition.
(arabesque is a kind of ballet position)
'sailor in lively square beginning to evolve' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'in' means one lot of letters goes inside another.
'lively' is an anagram indicator (I've seen this in other clues).
'beginning to' suggests taking the first letters.
The first letter of 'evolve' is 'e'.
'square' anagrammed gives 'aresqu'.
'ab' placed within 'aresqu' is 'arabesqu'.
'arabesqu'+'e'='ARABESQUE'
'of' is the link.
(Other definitions for arabesque that I've seen before include "Move for dance" , "posture only half-supported" , "Ballet position with one leg raised" , "Ballet position - sea barque (anag)" , "A square be a ballet movement?" .)