I go back and forth, write, and occasionally double up with a hint of mirth (8)
I believe the answer is:
pendulum
'i go back and forth' is the definition.
I can't tell whether this definition defines the answer.
'write and occasionally double up with a hint of mirth' is the wordplay.
'write' becomes 'pen' (synonyms).
'and' is a charade indicator (letters next to each other).
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'with' is a charade indicator (letters next to each other).
'a hint of' says to take the initial letters (the smallest possible amount of the word).
The initial letter of 'mirth' is 'm'.
'double'+'up'='doubleup'
The alternate letters of 'doubleup' are 'dulu'.
'pen'+'dulu'+'m'='PENDULUM'
Can you help me to learn more?
(Other definitions for pendulum that I've seen before include "A swinging weight as in old clock" , "It swings to and fro as in old clock" , "Moves to and fro" , "Weight hung from a fixed point to regulate a clock" , "Old time swinger?" .)