Pact partners initially will pen concerning new union (6)
I believe the answer is:
prenup
'pact partners initially' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'will pen concerning new union' is the wordplay.
'will' becomes 'pp' (I can't explain this - if you can you should give a lot more credence to this answer).
'pen' means one lot of letters goes inside another.
'concerning' becomes 're'.
'new' becomes 'n'.
'union' becomes 'u' (common abbreviation as in 'Trades Union Congress').
're'+'n'+'u'='renu'
'pp' enclosing 'renu' is 'PRENUP'.
Can you help me to learn more?
(Other definitions for prenup that I've seen before include "Contract between those about to marry" , "Financial agreement in case of divorce made by a couple before marriage (abbr)" , "Legal agreement before marriage" , "Agreement drawn up before the wedding" , "Contract before marriage, in short" .)