Dog occupying empty pen not at all clean (6)
I believe the answer is:
smutty
'at all clean' is the definition.
I can't tell whether this definition defines the answer.
'dog occupying empty pen' is the wordplay.
'dog' becomes 'mutt' (mutt is a kind of dog).
'occupying' means one lot of letters goes inside another (inserted letters occupy or enter the word).
'empty' suggests removing the centre.
'pen' becomes 'sty' (both can mean an animal enclosure).
'sty' with its centre removed is 'sy'.
'mutt' inserted into 'sy' is 'SMUTTY'.
'not' acts as a link.
Can you help me to learn more?
(Other definitions for smutty that I've seen before include "Dirty" , "Blue" , "Rather indecent" , "Improper" , "Filthy" .)