Plant pip containing source of protein before spring (9)
I believe the answer is:
speedwell
'plant' is the definition.
(speedwell is a kind of plant)
'pip containing source of protein before spring' is the wordplay.
'pip' becomes 'seed' (I've seen this before).
'containing' is an insertion indicator.
'source of' indicates taking the first letters.
'before spring' becomes 'well' (both can mean a source of water. I am not sure about the 'before' bit.).
The first letter of 'protein' is 'p'.
'seed' placed around 'p' is 'speed'.
'speed'+'well'='SPEEDWELL'
(Other definitions for speedwell that I've seen before include "Small plant, veronica" , "bloomer?" , "Small creeping herbaceous plant with blue or pink flowers" , "Small blue flower of the genus veronica" , "Flowering plant" .)