Repeat base expression of Scotsman dropping ball (4)
I believe the answer is:
echo
'repeat' is the definition.
(synonyms)
'base expression of scotsman dropping ball' is the wordplay.
I cannot quite understand how this works, but
'base' could be 'e' (mathematical symbol) and 'e' is found within the answer.
'of' could be 'o' and 'o' is found in the answer.
The remaining letters 'ch' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for echo that I've seen before include "Radio operator's word for 'E'" , "The distant sound" , "repercussion" , "Response" , "Before foxtrot" .)