Roots developing into a trunk (5)
I believe the answer is:
torso
'a trunk' is the definition.
('torso' can be a synonym of 'trunk')
'roots developing' is the wordplay.
'developing' indicates anagramming the letters.
'roots' is an anagram of 'TORSO'.
'into' acts as a link.
(Other definitions for torso that I've seen before include "Body (excluding head, neck and limbs)" , "Body's trunk" , "unfinished work" , "part of body will be found" , "Trunk; headless, limbless body" .)