Lake volunteers for the navy (4)

I believe the answer is:

'lake' is the definition.
(tarn is a kind of lake)

'volunteers for the navy' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'for' means one lot of letters go next to another.
'the navy' becomes 'RN' (abbreviation for Royal Navy).
'ta'+'rn'='TARN'

(Other definitions for tarn that I've seen before include "High water" , "Scottish mountain lake" , "Mountain pool: French department" , "Small mountain lake" .)

I've seen this clue in the Evening Standard.

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