Spike in rent around end of month (5)
I believe the answer is:
thorn
'spike' is the definition.
(I've seen this before)
'rent around end of month' is the wordplay.
'rent' becomes 'torn' (synonyms).
'around' indicates putting letters inside.
'end of' suggests the final letters.
The final letter of 'month' is 'h'.
'torn' going around 'h' is 'THORN'.
'in' acts as a link.
(Other definitions for thorn that I've seen before include "pain in one's side" , "(Rose's) prickle" , "Letter written long ago" , "Sharp point on, eg, rose" , "Cause of grief" .)