Part of body shape swollen by organ (7)
'part of body' is the definition.
(I've seen this before)
'shape swollen by organ' is the wordplay.
'shape' becomes 'form' (synonyms).
'swollen by' is an insertion indicator (the word's size swells after insertion).
'organ' becomes 'ear' (ear is a kind of organ).
'form' going around 'ear' is 'FOREARM'.
(Other definitions for forearm that I've seen before include "Attention", "Prepare to defend", "prepare defence in advance", "where is the radius?", "Before battle, take up weapon".)