Distressing detail about poor nag (8)
I believe the answer is:
poignant
'distressing' is the definition.
(I've seen this before)
'detail about poor nag' is the wordplay.
'detail' becomes 'point' (synonyms).
'about' is an insertion indicator.
'poor' indicates anagramming the letters.
'nag' is an anagram of 'gna'.
'point' enclosing 'gna' is 'POIGNANT'.
(Other definitions for poignant that I've seen before include "Sharply, painfully emotive" , "Moving, touching" , "sad" , "Sharply upsetting" , "pathetic" .)