Head in charge of lesson's theme? (5)
I believe the answer is:
topic
'lesson's theme?' is the definition.
(theme for a lesson)
'head in charge' is the wordplay.
'head' becomes 'top' (synonyms).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'top'+'ic'='TOPIC'
'of' acts as a link.
(Other definitions for topic that I've seen before include "'Theme, subject (5)'" , "Subject (for conversation, etc)" , "Subject matter, theme" , "Content of argument" , "Issue (for discussion)" .)