Testy poses deter worst poser of this, for one (9,6)
I believe the answer is:
crossword setter
'poser of this for one' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'testy poses deter worst' is the wordplay.
'testy' becomes 'cross' (I've seen this in another clue).
'poses' indicates an anagram (I've seen 'pose' mean this (the letters must be re-posed into a new order)).
'deter'+'worst'='deterworst'
'deterworst' is an anagram of 'wordsetter'.
'cross'+'wordsetter'='CROSSWORD SETTER'
Can you help me to learn more?
(Other definitions for crossword setter that I've seen before include "Provider of puzzles" , "Punk, perhaps" , "one making you suffer?" , "In this case, yours truly" .)