The sailor dined with the guys before starting the decrease (9)
I believe the answer is:
abatement
'decrease' is the definition.
(abatement is a kind of decrease)
'the sailor dined with the guys before starting the' is the wordplay.
'the sailor' becomes 'ab' (abbreviation for able seaman).
'dined with' becomes 'ate' (I've seen this in another clue. I am not sure about the 'with' bit.).
'the guys' becomes 'men' (guy can mean a man).
'before' means one lot of letters go next to another.
'starting' suggests taking the first letters.
The initial letter of 'the' is 't'.
'ab'+'ate'+'men'+'t'='ABATEMENT'
Can you help me to learn more?
(Other definitions for abatement that I've seen before include "mitigation" , "relief" , "Subsidence" , "Lessening in intensity or amount" .)