Maintain right to invest in something of value (6)
I believe the answer is:
assert
'maintain' is the definition.
(synonyms)
'right to invest in something of value' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to invest in' means one lot of letters goes inside another.
'something of value' becomes 'asset' (I've seen this before).
'r' put within 'asset' is 'ASSERT'.
(Other definitions for assert that I've seen before include "Aver, maintain" , "State firmly and with conviction" , "Affirm, allege" , "Establish control or state as true" , "Declare, state vigorously" .)