Four-letter word from guy probing complex matter (9)
I believe the answer is:
tetragram
'four-letter word' is the definition.
(I've seen this before)
'guy probing complex matter' is the wordplay.
'guy' becomes 'rag' (both can mean to tease or mock).
'probing' indicates putting letters inside (probe can mean to make a hole in).
'complex' indicates an anagram.
'matter' is an anagram of 'tetram'.
'rag' placed into 'tetram' is 'TETRAGRAM'.
'from' is the link.
(Other definitions for tetragram that I've seen before include "Four-letter word" , "familiar to geometricians !" , "Say damn (but not blast)" .)