Suggest in straightforward manner son must leave (5)
I believe the answer is:
imply
'suggest' is the definition.
(implying is a kind of suggesting)
'in straightforward manner son must leave' is the wordplay.
'in straightforward manner' becomes 'simply'.
'son' becomes 's'.
'must leave' suggests deleting specific letters.
'simply' with 's' taken away is 'IMPLY'.
(Other definitions for imply that I've seen before include "Involve - intimate" , "Indicate indirectly" , "Insinuate or suggest" , "Indicate by suggestion" , "Hint, suggest" .)