Second gear is curse (5)
I believe the answer is:
swear
'is curse' is the definition.
(I've seen this before)
'second gear' is the wordplay.
I cannot quite understand how this works, but
'second' could be 's' (common abbreviation) and 's' is present in the answer.
The remaining letters 'wear' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for swear that I've seen before include "Solemnly promise" , "Promise on oath" , "not to mince one's words" , "Eff and blind" , "Asseverate" .)