In plants uranium has leaked across – it’s all over! (6)
I believe the answer is:
tulips
'in plants' is the definition.
'tulips' can be an answer for 'plants' (I've seen this before). I am not certain of the 'in' bit.
'uranium has leaked across it's all over' is the wordplay.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'has' says to put letters next to each other.
'leaked across' becomes 'lip' (I am not sure about this - if you are sure you should believe this answer much more).
'all over' is an insertion indicator.
'u'+'lip'='ulip'
'ulip' going within 'ts' is 'TULIPS'.
Can you help me to learn more?
(Other definitions for tulips that I've seen before include "Nice flowers, musically from Amsterdam" , "Garden flowers - tip-toe through them in song" , "Garden flowers, musically from Amsterdam" , "Pretty flowers, musically from Amsterdam" , "Perennial bulbs" .)