Surprisingly, it was the narrow part! (5)
I believe the answer is:
waist
'narrow part' is the definition.
(I've seen this before)
'surprisingly it was' is the wordplay.
'surprisingly' indicates an anagram (a surprising ordering of the letters).
'it'+'was'='itwas'
'itwas' is an anagram of 'WAIST'.
'the' acts as a link.
(Other definitions for waist that I've seen before include "Ever-expanding middle" , "Middle of trunk" , "part of frame" , "Central part of body" , "Narrow part of body" .)