Wind power extremely handy, almost nothing eclipses it (6)
'wind' is the definition.
(zephyr is a kind of wind)
'power extremely handy almost nothing eclipses it' is the wordplay.
'power' becomes 'p'.
'extremely' says to hollow out the word (remove centre letters).
'almost' means to remove the last letter (most of the word but not all of it).
'nothing' becomes 'zero' (synonyms).
'eclipses it' means one lot of letters goes inside another (to eclipse can mean to cover up).
'handy' with its middle taken out is 'hy'.
'zero' with its last letter removed is 'zer'.
'phy' put inside 'zer' is 'ZEPHYR'.
(Other definitions for zephyr that I've seen before include "Slight breeze", "A light gentle breeze", "A soft gentle breeze", "Light wind", "Greek god of the west wind".)