Like a formal photo of a police officer? (7)

I believe the answer is:

mounted

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'like' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

I don't understand the rest of the clue.

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(Other definitions for mounted that I've seen before include "Riding a horse" , "riding horses" , "On horseback" , "On a steed" , "Scaled" .)

I've seen this clue in the Evening Standard.

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