Sudden insight: northbound Tube convenient when dropping daughter (8)
I believe the answer is:
epiphany
'sudden insight' is the definition.
(I've seen this before)
'northbound tube convenient when dropping daughter' is the wordplay.
'northbound' says the letters should be written backwards.
'tube' becomes 'pipe' (pipes are tubular).
'convenient' becomes 'handy' (similar in meaning).
'when dropping' suggests deleting specific letters.
'daughter' becomes 'd' (genealogical abbreviation).
'pipe' written backwards gives '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" .)