Trusty leaders of local organisation backed nonprofessional (5)
I believe the answer is:
Here is my best explanation:
'trusty' is the definition.
'leaders of local organisation backed nonprofessional' is the wordplay.
'leaders of' indicates taking the first letters.
'backed' says the letters should be written in reverse.
'nonprofessional' becomes 'lay' ('lay' can be a synonym of 'nonprofessional').
The first letters of 'local organisation' is 'lo'.
'lay' backwards is 'yal'.
(Other definitions for loyal that I've seen before include "Steadfast and true", "Unwaveringly devoted to friend or cause", "firm in solidarity", "Sort of toast", "Faithful to country, cause or friend", "Keeping faith", "True-hearted", "True and faithful", "Not perfidious" and "Showing firm allegiance".)