One devoted to single subject captured by king (4)

I believe the answer is:

wonk

I'm a little stuck... Click here to teach me more about this clue!

'one devoted to single subject' is the definition.
I can't tell whether this defines the answer.

'captured by king' is the wordplay.
'captured' becomes 'won' (I've seen this in another clue).
'by' means one lot of letters go next to another.
'king' becomes 'K' (chess abbreviation).
'won'+'k'='WONK'

Can you help me to learn more?

I've seen this clue in The Irish Times.

Want a hint initially instead of a full solution? Install my app