Play for cash, initially beside Welsh river (4)
I believe the answer is:
busk
'play' is the definition.
(I know this)
'initially beside welsh river' is the wordplay.
'initially' says to take the initial letters.
'welsh river' becomes 'usk' (I've seen this before).
The initial letter of 'beside' is 'b'.
'b'+'usk'='BUSK'
'for cash' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for busk that I've seen before include "Improvise; perform for money in the street" , "Play for donations" , "Play an instrument on the street for money" , "Perform, seeking coins" , "provide entertainment for money" .)