The alinment of the shifter rods are bent
The big rubber golf ball bush is worn
Somewhere on the exhaust is loose and banging on the selector rods creating mis-alinement.
Fifth gear has shat itself
Clutch bearing or clutch rod is done.
One Loose clutch bolts
Clutch cable tension loose.
There are a few more but its a dark art the fifth gear I've done four out of the seven and told a mate to change the gear box out when I gave up