Little flower that's extremely useful held by joiner (7)
I believe the answer is:
rivulet
'little flower' is the definition.
'rivulet' can be an answer for 'flower' (thesaurus). I'm not sure about the 'little' bit.
'extremely useful held by joiner' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'held by' is an insertion indicator.
'joiner' becomes 'rivet' (I am not sure about this - if you are sure you should believe this answer much more).
'useful' with its centre taken out is 'ul'.
'ul' placed inside 'rivet' is 'RIVULET'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for rivulet that I've seen before include "fast stream" , "Vile rut for small stream" , "Brook" , "Big stream" , "burn" .)