I have to get right outside to get the pin (5)
I believe the answer is:
'the pin' is the definition.
(rivet is a kind of pin)
'i have to get right outside' is the wordplay.
'i have' becomes 'I've' (contraction of 'I have').
'to get' indicates putting letters inside.
'outside' suggests removing the centre.
'right' with its centre removed is 'rt'.
'ive' put within 'rt' is 'RIVET'.
'to get' acts as a link.
(Other definitions for rivet that I've seen before include "Clinch - bolt" , "Short metal fastener" , "Captivate" , "Type of fastening" , "Heavy fastening pin with head and hammered end" .)