Third attempt to hold it back in time (8)
I believe the answer is:
tertiary
'time' is the definition.
(Tertiary is an example)
'third attempt to hold it back in' is the wordplay.
'third' becomes 'era' (I am not sure about this - if you are sure you should believe this answer much more).
'attempt' becomes 'try' (synonyms).
'to hold' means one lot of letters goes inside another.
'back' shows that the letters should be reversed in order.
'in' is an insertion indicator.
'it' written backwards gives 'ti'.
'era' going within 'try' is 'terary'.
'terary' enclosing 'ti' is 'TERTIARY'.
Can you help me to learn more?
(Other definitions for tertiary that I've seen before include "Of the third rank, after secondary" , "Third in order or degree" , "very long period" , "Third; period of prehistory" , "Of the third rank or stage" .)