Bones, when like that, turned over (4)
I believe the answer is:
ossa
'bones' is the definition.
(I've seen this before)
'when like that turned over' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'like that' becomes 'so' (as in 'like so').
'turned over' is a reversal indicator.
'as'+'so'='asso'
'asso' reversed gives 'OSSA'.
(Other definitions for ossa that I've seen before include "Greek mountains" , "To pile Pelion upon - is to make even greater" , "To heap Pelion on -- is to compound difficulties" , "mountain in Thessaly" , "Technical name for bones" .)