First son upset in nightclub (4)
I believe the answer is:
spot
'nightclub' is the definition.
'first son upset' is the wordplay.
'first' becomes 'top' (top can mean the best or first).
'son' becomes 's' (genealogical abbreviation for son).
'upset' is a reversal indicator.
'top'+'s'='tops'
'tops' written backwards gives 'SPOT'.
'in' is the link.
(Other definitions for spot that I've seen before include "See - blemish" , "Notice - stain - pick out" , "Dot, stain" , "Lady Macbeth: ''Out, damned . . . .!''" , "See; location" .)