Should have got them off first, I reckoned (9)
I believe the answer is:
methought
'should have got them off' is the definition.
I can't tell whether this definition defines the answer.
'first i reckoned' is the wordplay.
'first' means one lot of letters go next to another.
'i' becomes 'me'.
'reckoned' becomes 'thought' ('think' can be a synonym of 'reckon').
'me' put next to 'thought' is 'METHOUGHT'.
Can you help me to learn more?
(Other definitions for methought that I've seen before include "The speaker concluded" , "I had the impression" , "In old-fashioned way, I considered" .)