Bulls nibbling tips of yellow grass — a source of energy (6)

I believe the answer is:

oxygen

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(Other definitions for oxygen that I've seen before include "Gas that combines with hydrogen to form water" , "Element essential to combustion" , "Life supporting element" , "Colourless, odourless, tasteless non-flammable gas" , "With hydrogen it forms water" .)

I've seen this clue in The Hindu.

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