Don't know what to say? It's no way to make a sale (2,1,4)
I believe the answer is:
at a loss
'don't know what to say?' is the definition.
I can't tell whether this definition defines the answer.
'it's no way to make a sale' is the wordplay.
I cannot quite see how this works, but
'no' could be 'o' (O resembles 0) and 'o' is found within the answer.
'way' could be 's' (Compass direction - South) and 's' is located in the answer.
'a' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
The remaining letters 'als' 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 at a loss that I've seen before include "Unable to understand - no profit in it" , "(Running) in deficit" , "Completely puzzled, with no idea at all" , "How unprofitable business is run" , "Puzzled, bewildered" .)