Third rock's troubles irreversible, first of all: very sad about that (8)
I believe the answer is:
tertiary
'third' is the definition.
(I know that third can be written as tertiary)
'rock's troubles irreversible first of all very sad about that' is the wordplay.
'first of all' indicates taking the first letters.
'very sad' becomes 'teary' (I've seen this in another clue).
'about that' indicates putting letters inside.
The first letters of 'rocks troubles irreversible' is 'rti'.
'rti' placed into 'teary' is 'TERTIARY'.
(Other definitions for tertiary that I've seen before include "Third in order or degree" , "Try a rite (anag.)" , "Of the third rank or stage" , "ancient period" , "Of the third rank, after secondary" .)