Returned letter I afterwards get back (9)
I believe the answer is:
retaliate
'get back' is the definition.
The definition and answer can be both related to competing as well as being verbs in their base form.
Perhaps you can see an association between them that I can't see?
'returned letter i afterwards' is the wordplay.
'returned' shows that the letters should be reversed in order.
'letter' becomes 'eta' (eta is a kind of letter of the alphabet).
'afterwards' becomes 'later' (synonyms).
'eta'+'i'+'later'='etailater'
'etailater' in reverse letter order is 'RETALIATE'.
Can you help me to learn more?
(Other definitions for retaliate that I've seen before include "Take revenge for a perceived wrong" , "Fight back" , "Strike back, avenge" , "Act in revenge" , "Strike back, counter" .)