Serious as soldiers (7)

I believe the answer is:

sincere

'serious' is the definition.
('sincere' can be similar in meaning to 'serious')

'as soldiers' is the wordplay.
'as' becomes 'since' (I've seen this before).
'soldiers' becomes 're' (Royal Engineers).
'since'+'re'='SINCERE'

(Other definitions for sincere that I've seen before include "Free from deception" , "Genuine and honest" , "Free from deceit" , "devout" , "Lacking in deceit" .)

I've seen this clue in The Independent.

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