Three also involved become disillusioned (4,5)
'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" .)