It picked up socialist on loch showing fatigue (9)
I believe the answer is:
tiredness
'fatigue' is the definition.
(I know that fatigue can be written as tiredness)
'it picked up socialist on loch' is the wordplay.
'picked up' says the letters should be written in reverse.
'socialist' becomes 'red' (socialism is often associated with the colour red).
'on' is a charade indicator (letters next to each other).
'loch' becomes 'ness' (Loch Ness in Scotland).
'it' in reverse letter order is 'ti'.
'ti'+'red'+'ness'='TIREDNESS'
'showing' is the link.
(Other definitions for tiredness that I've seen before include "Exhaustion" , "Weary feeling" , "Lassitude; residents (anag.)" , "Residents in state of fatigue" , "That's liable to occur at midnight" .)