Thanks to the navy, one might sail on it (4)
I believe the answer is:
tarn
'one might sail on it' is the definition.
I can't judge whether this defines the answer.
'thanks to the navy' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'to the navy' becomes 'RN' (abbreviation for Royal Navy).
'ta'+'rn'='TARN'
Can you help me to learn more?
(Other definitions for tarn that I've seen before include "Scottish mountain lake" , "Mountain pool: French department" , "Small mountain lake" , "High water" .)