Make use of one in the union (5)
I believe the answer is:
wield
'make use of' is the definition.
(I've seen this in another clue)
'one in the union' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
The remaining letters 'weld' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for wield that I've seen before include "Brandish a weapon or exert power" , "Handle - manage" , "Hold and use (a weapon or tool)" , "Use (weapon, authority)" , "exercise" .)