Engineers like this instrument firm (8)

I believe the answer is:

resolute

'firm' is the definition.
('resolute' can be similar in meaning to 'firm')

'engineers like this instrument' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'like this' becomes 'so'.
'instrument' becomes 'lute' (lute is a kind of instrument).
're'+'so'+'lute'='RESOLUTE'

(Other definitions for resolute that I've seen before include "'Firm, unhesitating (8)'" , "Firmly determined" , "'Determined, dead set on (8)'" , "in a determined manner" , "without flinching" .)

I've seen this clue in The Telegraph.

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