Was GI Joe's head turned? That's a puzzle (6)
I believe the answer is:
jigsaw
'that's a puzzle' is the definition.
(jigsaw puzzle is a type of puzzle)
'was gi joe's head turned?' is the wordplay.
'head' suggests taking the first letters.
'turned?' says the letters should be written backwards.
The initial letter of 'joes' is 'j'.
'was'+'gi'+'j'='wasgij'
'wasgij' reversed gives 'JIGSAW'.
(Other definitions for jigsaw that I've seen before include "Puzzle-maker" , "Puzzle fittingly completed" , "Sharp tool (for cutting puzzle into pieces?)" , "Puzzle - powered cutter" , "Woodcutter" .)