noun
nearly one hour; about an hour
Used to indicate a duration slightly less than an hour, often with a sense of 'a good part of an hour' or 'the better part of an hour'.
小一時間ほど散歩した。
I took a walk for nearly an hour.
小一時間で着くと思います。
I think we'll arrive in about an hour.
一時間 means exactly one hour, while 小一時間 means nearly one hour (a little less than an hour).
Compound of 小 (ko, 'small') + 一時間 (ichijikan, 'one hour'). The prefix 小- softens the meaning to 'a little less than one hour'.