Clue: One after something yellow primarily in flower, a shrub (9)
Answer: FORSYTHIA
Explanation: Definition: shrub
An envelope ('in') of SY ('**S**omething **Y**ellow primarily') in FORTH (Scottish river, 'flower') plus I ('one', with 'after' indicating its position in the answer) plus 'a' (with its position in the clue indicating its position in the answer), with an extended definition.