Queen of mystery: queen wearing boot but no cap (6)
I believe the answer is:
ellery
'queen of mystery' is the definition.
(mystery author Ellery Queen)
'queen wearing boot but no cap' is the wordplay.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'wearing' means one lot of letters goes inside another.
'boot' becomes 'welly' (short for Wellington boot).
'but no cap' means to remove the first letter (cap can mean the top letter).
'welly' with its initial letter taken away is 'elly'.
'er' going within 'elly' is 'ELLERY'.
(Another definition for ellery that I've seen is " - - Queen (detective fiction)".)