Cryptic crossword clue
Substitute for gas boiler’s top half working with flue 7

The most common and most likely answer to this crossword clue is the 7 letter word BIOFUEL

I've seen this crossword clue in The Guardian
B
I
O
F
U
E
L

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The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.
This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.

What does BIOFUEL mean?

BIOFUEL noun
  1. Any fuel that is obtained from a renewable biological resource, especially from biomass

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