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) (letters on the extremes of the word are used).
'held by' means one lot of letters goes inside another.
'joiner' becomes 'rivet' (I can't justify this - if you can you should believe this answer much more).
'useful' with its centre taken out is 'ul'.
'ul' going within 'rivet' is 'RIVULET'.
'that's' is the 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" .)