One left enthralled by friend's physique (5)
I believe the answer is:
build
'physique' is the definition.
(I know that physique can be written as build)
'one left enthralled by friend's' is the wordplay.
'one' becomes 'i' (Roman numeral).
'left' becomes 'L' (common abbreviation).
'enthralled by' indicates putting letters inside ('enthral' can mean 'capture').
'friend' becomes 'bud' (shortly for buddy).
'i'+'l'='il'
'il' going into 'bud' is 'BUILD'.
(Other definitions for build that I've seen before include "Construct" , "Institute" , "Create" , "Increase gradually" , "Put up" .)