Initially ten, approximately, in trunk (5)
I believe the answer is:
torso
'trunk' is the definition.
('torso' can be a synonym of 'trunk')
'initially ten approximately' is the wordplay.
'initially' says to take the initial letters.
'approximately' becomes 'orso' ('or so' can be a synonym of 'approximately').
The first letter of 'ten' is 't'.
't'+'orso'='TORSO'
'in' is the link.
(Other definitions for torso that I've seen before include "Body's trunk" , "Roost (anag)" , "Trunk of human body" , "Body, minus its head and limbs" , "Chest" .)