What happens to the allele frequency when opposing selective forces are acting on an allele?

It all depends if those forces are acting on the allele equally or not.  If they are, the allele frequency (and therefore the genotype and phenotype frequency) will not change.  If the opposing forces are both acting, but in an unequal way, then yes, the allele frequency will change.  The direction of the change (towards higher or lower allele frequency) will determine genotype and phenotype frequency changes.

This is why it is good to point out that there are many selective forces at work on a population at the same time (natural selection, sexual selection, gene flow, migration, mutation, genetic drift), and it is the NET outcome of those forces that determines an evolutionary outcome.