Summon but not on stream (4)
I believe the answer is:
beck
'stream' is the definition.
(northern English dialect term for 'stream')
'summon but not on' is the wordplay.
'summon' becomes 'beckon' (beckoning is a kind of summoning).
'but not' indicates named letters should be taken away.
'beckon' with 'on' taken away is 'BECK'.
(Other definitions for beck that I've seen before include "Stream (N. England)" , "Northern stream" , "swiftly flowing stream?" , "Fast stream; goes with call" , "Call for attention" .)