Drops first-class navy trousers (4)
I believe the answer is:
rain
'drops' is the definition.
(drops of water)
'first-class navy trousers' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'navy' becomes 'RN' (abbreviation for Royal Navy).
'trousers' indicates putting letters inside (to trouser can mean to steal or take).
'ai' inserted inside 'rn' is 'RAIN'.
(Other definitions for rain that I've seen before include "bad weather" , "Storm" , "Cats and dogs?" , "Precipitation" , "The ---, it ---eth every day (T.Night)" .)