Flight paid for by new partner's family (7)
I believe the answer is:
stepson
'new partner's family' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'flight paid for by' is the wordplay.
'flight' becomes 'steps' (I can't explain this - if you can you should believe this answer much more).
'paid for by' becomes 'on' (as in 'the drinks are on me').
'steps'+'on'='STEPSON'
Can you help me to learn more?
(Other definitions for stepson that I've seen before include "younger member of family" , "Tramples (5,2) -- relative" , "Spouse's child from previous marriage" , "Child of a spouse by a former marriage" , "Child from a spouse's former marriage" .)