Insufficient wood for a fire (3)

I believe the answer is:

fir

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

'insufficient wood' is the definition.
'fir' can be an answer for 'wood' (fir is a kind of wood). I am not sure about the 'insufficient' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for fir that I've seen before include "Coniferous tree" , "Pine tree for Gaelic men?" , "Tree for an Irishman" , "Scots pine" , "Conifer tree" .)

I've seen this clue in the Evening Standard.

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