Has forty winks up in the bridge (4)
I believe the answer is:
span
'bridge' is the definition.
(synonyms)
'has forty winks up' is the wordplay.
'has' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'forty winks' becomes 'nap' (synonyms).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'nap' written backwards gives 'pan'.
's'+'pan'='SPAN'
'in the' is the link.
Can you help me to learn more?
(Other definitions for span that I've seen before include "Extent of 20" , "Full extent end to end" , "Stretch < fresh" , "Steeleye ___ , British folk-rock band, formed 1969" , "Duration, length" .)