It explains the meaning of occasional steamy mischief (9)
I believe the answer is:
semantics
'it explains the meaning' is the definition.
The definition suggests a singular noun which matches the answer.
'occasional steamy mischief' is the wordplay.
'occasional' means one should take alternating letters (letters taken at regular intervals or on occasion).
'mischief' becomes 'antics' (I can't explain this - if you can you should give a lot more credence to this answer).
The alternating letters of 'steamy' are 'sem'.
'sem'+'antics'='SEMANTICS'
'of' is the link.
Can you help me to learn more?
(Other definitions for semantics that I've seen before include "Branch of linguistics concerned with meanings" , "Meaning to study" , "The meaning attached to words" , "The significance of a text" , "Branch of linguistics to do with meaning" .)