Instrument in boat beginning to monitor polluted water (10)
I believe the answer is:
tambourine
'instrument' is the definition.
(I know that tambourine is a type of drum)
'boat beginning to monitor polluted water' is the wordplay.
'beginning to' indicates taking the first letters.
'polluted' indicates an anagram.
'water' becomes 'urine' (synonyms).
The initial letter of 'monitor' is 'm'.
'boat'+'m'='boatm'
'boatm' anagrammed gives 'tambo'.
'tambo'+'urine'='TAMBOURINE'
'in' acts as a link.
(Other definitions for tambourine that I've seen before include "Percussion instrument" , "Shaken instrument" , "drum" , "One uses jingles" .)