Unfortunately upset catching son dressing (5)
I believe the answer is:
salsa
'dressing' is the definition.
(I've seen this before)
'unfortunately upset catching son' is the wordplay.
'unfortunately' becomes 'alas' ('alas' is used to express regret).
'upset' says the letters should be written backwards.
'catching' indicates putting letters inside.
'son' becomes 's' (genealogical abbreviation for son).
'alas' backwards is 'sala'.
'sala' going around 's' is 'SALSA'.
(Other definitions for salsa that I've seen before include "hot stuff" , "fling?" , "that's hot and saucy?" , "Dance or a dip?" , "Chilli sauce" .)