Sick postie keeping uniform is poignant (7)
I believe the answer is:
piteous
'poignant' is the definition.
(thesaurus)
'sick postie keeping uniform' is the wordplay.
'sick' indicates anagramming the letters.
'keeping' indicates putting letters inside (some letters kept inside).
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'postie' anagrammed gives 'piteos'.
'piteos' placed around 'u' is 'PITEOUS'.
'is' is the link.
(Other definitions for piteous that I've seen before include "Heart-rending" , "Deserving sympathy" , "Woeful" , "Arousing sorrow and compassion" , "Deserving compassion" .)