Cover up of certain features of Cromwell's reversal at Derby? (5,3)
I believe the answer is:
straw hat
'cover up' is the definition.
(cover worn up on one's head)
'certain features of cromwell's reversal at derby?' is the wordplay.
'certain features of cromwell's' becomes 'warts' (Oliver Cromwell famously had warts).
'reversal' says the letters should be written in reverse.
'at' is a charade indicator (letters next to each other).
'derby?' becomes 'hat' (derby is a kind of hat).
'warts' back-to-front is 'straw'.
'straw'+'hat'='STRAW-HAT'
'of' is the link.
(Other definitions for straw hat that I've seen before include "Boater" , "Panama" , "Capital Investment?"" .)