Beloved harbouring fear finally becomes dull (5)
I believe the answer is:
drear
'dull' is the definition.
(I've seen this before)
'beloved harbouring fear finally' is the wordplay.
'beloved' becomes 'dear' ('dear' can be a synonym of 'beloved').
'harbouring' indicates putting letters inside.
'finally' suggests the final letters.
The last letter of 'fear' is 'r'.
'dear' going around 'r' is 'DREAR'.
'becomes' acts as a link.
(Other definitions for drear that I've seen before include "Morose" , "Depressingly dull" , "gloomy in poem" , "Dull, bleak, depressing" , "Dismal (literary)" .)