The plant, a weed, spreads in a while (9)
I believe the answer is:
speedwell
'the plant' is the definition.
(speedwell is a kind of plant)
'a weed spreads in a while' is the wordplay.
'spreads' is an anagram indicator (I've seen 'spreading of' mean this).
'in' indicates putting letters inside.
'a while' becomes 'spell' (I've seen this in another clue).
'weed' anagrammed gives 'eedw'.
'eedw' placed within 'spell' is 'SPEEDWELL'.
(Other definitions for speedwell that I've seen before include "Small creeping herbaceous plant with blue or pink flowers" , "Small plant, veronica" , "Small blue flower of the genus veronica" , "bloomer?" , "Flowering plant" .)