Build up rents incorrectly, afterwards following initial guideline (10)
I believe the answer is:
strengthen
'build up' is the definition.
(I've seen this before)
'rents incorrectly afterwards following initial guideline' is the wordplay.
'incorrectly' indicates anagramming the letters (I've seen this in other clues).
'afterwards' becomes 'then' (I've seen this in other clues).
'following' is a charade indicator (letters next to each other).
'initial' suggests taking the first letters.
The first letter of 'guideline' is 'g'.
'rents' with letters rearranged gives 'stren'.
'then' put after 'g' is 'gthen'.
'stren'+'gthen'='STRENGTHEN'
(Other definitions for strengthen that I've seen before include "Make tough" , "Build up" , "Brace" , "Fortify" , "rally" .)