One showing off for all to see, cracking tough puzzle (6)
I believe the answer is:
poseur
'one showing' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'for all to see cracking tough puzzle' is the wordplay.
'for all' becomes 'U' (Universal film certificate - suitable for all).
'to see cracking' indicates putting letters inside (inserted letters go into a gap or crack).
'tough puzzle' becomes 'poser' (I've seen this in another clue).
'u' inserted inside 'poser' is 'POSEUR'.
'off' is the link.
Can you help me to learn more?
(Other definitions for poseur that I've seen before include "would-be know-all" , "One who strikes attitudes, a phoney" , "Pretentious person" , "Hypocrite!" , "Affected type" .)