Sudden insight: northbound Tube convenient when dropping daughter (8)
I believe the answer is:
epiphany
'sudden insight' is the definition.
(I've seen this in another clue)
'northbound tube convenient when dropping daughter' is the wordplay.
'northbound' is a reversal indicator.
'tube' becomes 'pipe' (pipes are tubular).
'convenient' becomes 'handy' (synonyms).
'when dropping' indicates named letters should be taken away.
'daughter' becomes 'd' (genealogical abbreviation).
'pipe' in reverse letter order is 'epip'.
'handy' with 'd' taken away is 'hany'.
'epip'+'hany'='EPIPHANY'
(Other definitions for epiphany that I've seen before include "A moment of sudden understanding or revelation" , "Its events occur in Matthew" , "Moment of sudden revelation (involving three wise men from the east?)" , "Sudden insight" , "Christian Festival, showing of Christ to the Magi" .)