Tiny tip one’s left in main plant (5,3)
I believe the answer is:
Here is my best explanation:
'plant' is the definition.
(sweet pea is a kind of plant)
'tiny tip one's left in main' is the subsidiary indication.
'tiny' becomes 'wee' (I've seen this before).
'one's' becomes 'i' (I've seen this in other clues).
'left' indicates named letters should be taken away.
'in' is an insertion indicator.
'main' becomes 'sea' ('sea' can be a synonym of 'main'**).
'tip' with 'i' taken out is 'tp'.
'weetp' inserted into 'sea' is 'sweetpea'.
(Other definitions for sweet pea that I've seen before include "Pew tease (anag) - flower", "Climbing garden plant - saw tepee (anag)", "Fragrant plant", "flowering plant", "Peseta we paid for climbing garden flower", "Climbing plant with pastel-coloured flowers", "Climbing plant with fragrant flowers", "Celestial source of fluctuating radio waves", "Eat, weeps and gets climbing flowers" and "Flower".)