The most common and most likely answer to this crossword clue is the 9 letter word INSOMNIAC
I've seen this crossword clue in Independent
I
N
S
O
M
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A
C
More clues leading to the result INSOMNIAC
- One can't go off at nights
- Wakeful type in county recalled famous murderer
- One manic son unfortunately is not sleeping
- I often can’t go out when I want
- One often up probably wasting money in casino
- Simon’s gone crazy and turned killer and he can’t sleep at night
- Current issue involves money — first clue one probably won’t enjoy retirement
- One taken in by awful conman is suffering from sleep deficit
- One that’s unable to be out?
- One maybe needing number and coins, frantically, to ring Mina abroad
Buddy explains!

The straight part
We know that "I’m often unable to get off between the sheets" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i’m often unable to get off between the sheets = insomniac.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does INSOMNIAC mean?
INSOMNIAC noun- One who suffers an inability or difficulty sleeping; a sufferer from insomnia.
- Suffering from or pertaining to insomnia.