Hold firmly that I have to be in the right (5)
I believe the answer is:
rivet
'hold firmly' is the definition.
(I've seen this in another clue)
'i have to be in the right' is the wordplay.
'i have' becomes 'I've' (contraction of 'I have').
'to be in the' indicates putting letters inside.
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'ive' put within 'rt' is 'RIVET'.
'that' acts as a link.
(Other definitions for rivet that I've seen before include "Clinch - bolt" , "Metal fastening pin" , "Heavy fastening pin with head and hammered end" , "Short metal fastener" , "Fasten -- engross" .)