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I have to somehow get by on a small income.
Literal
I [topic-は] small income [with-で] somehow manage-going [must-なければならない].
やっていく ('to get by, manage, carry on') combines やる with ~ていく to express ongoing effort into the future. なんとか ('somehow, one way or another') is an adverb expressing coping under difficulty. ~なければならない adds obligation.