Three also involved become disillusioned (4,5)
![Ross](/clue/static/ross-mug.min.png)
'become disillusioned' is the definition.
(I've seen this before)
'three also involved' is the wordplay.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'three'+'also'='threealso'
'threealso' anagrammed gives 'LOSE HEART'.
(Other definitions for lose heart that I've seen before include "Here's a lot to cause courage to ebb" , "Become discouraged in cardiac manner?" , "find discouragement" , "What patient may do before subsequent transplant" , "get discouraged?" .)
'disillusioned' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for lost heart that I've seen before include "disillusioned" , "Despaired" , "Gave up" .)