Talk at length about GI's cryptic puzzle (6)
I believe the answer is:
jigsaw
'puzzle' is the definition.
(I know that puzzle can be written as jigsaw)
'talk at length about gi's cryptic' is the wordplay.
'talk at length' becomes 'jaw' (to jaw is to talk excessively).
'about' indicates putting letters inside.
'cryptic' indicates an anagram (a cryptic or obscure order of the letters).
'gis' is an anagram of 'igs'.
'jaw' going around 'igs' is 'JIGSAW'.
(Other definitions for jigsaw that I've seen before include "Woodcutter" , "Puzzle with pieces to assemble" , "Puzzle in which pieces must be reassembled" , "Puzzle - powered cutter" , "Type of puzzle" .)