Trend involved one large part of a plant (7)
I believe the answer is:
tendril
'a plant' is the definition.
The answer is an example.
'trend involved one large part' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'one' becomes 'i' (Roman numeral).
'part' says to take the initial letters.
The initial letter of 'large' is 'l'.
'trend' with letters rearranged gives 'tendr'.
'tendr'+'i'+'l'='TENDRIL'
'of' acts as a link.
(Other definitions for tendril that I've seen before include "Slender thread-like appendage of a plant" , "Twining shoot" , "Climbing plant's attachment" , "Climbing plant's gripper" , "Threadlike extension of a plant" .)