Pale, after engaging in drinking, doing work on the Thames (4-6)
I believe the answer is:
swan-upping
'doing work on the thames' is the definition.
I can't judge whether this definition defines the answer.
'pale after engaging in drinking' is the wordplay.
'pale' becomes 'wan' (wan can mean pale or grey).
'after engaging in' becomes 'supping' (I can't explain this - if you can you should believe this answer much more).
'drinking' means one lot of letters goes inside another (inserted letters are consumed or drunk).
'wan' put inside 'supping' is 'SWAN-UPPING'.
Can you help me to learn more?
(Other definitions for swan-upping that I've seen before include "Annual marking of Thames birds" , "Annual Thames event" .)